Seashells PDF - Mollusc Characteristics

Mollusca are usually characterized in the mind of man simply as "seashells." Malacology, the scientific study of molluscs as living organisms. Conchology the scientific study of shells. Conchologists, those people who study and collect shells, almost always study their animal makers- their anatomy,...

Mollusca are usually characterized in the mind of man simply as "seashells." Malacology, the scientific study of molluscs as living organisms. Conchology the scientific study of shells. Conchologists, those people who study and collect shells, almost always study their animal makers- their anatomy, life history, and habitats. Mollusc Habitats Both gastropods and bivalves inhabit rivers, streams, and lakes, where they have adapted to living in freshwater. Many species have also transitioned to land and can be found in leaf litter, on trees, on plants, under rocks, and buried in loose dirt. In ocean environments, mudflats and mangrove areas are homes for hundreds of mollusc species. Collecting shells sandy beaches intertidal mud and sand flats Subtidally 17  Reefs and rocky areas Rocky intertidal areas How to find seashells ? Beach walking Rocky shores exploration How to clean seashells ? Removing algae deposits Removing calcareous deposits And the same shells after beached Conus ebraeus After the removal of the bleach bath shells calcareus deposit that removed periostracum just after pick-up with a piece of metal and algae deposits If the deposits are too strong, it is possible to soften them using dilute acids or white vinegar. 18 Mollusc shell The mollusc shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca. A mollusc shell is formed by the mantle. Molluscs make their shells from calcium they derive from their environment, either the food they eat or the water they dwell in or the soil. Shell formation Molluscs can only form shells when they can extract CaCO3 (calcium carbonate) from the water, and keep it from being dissolved again. Reserve calcium carbonate is stored in certain cells of the digestive glad and is used for shell repair or to add new growth thus enlarging the shell for the growing animal. Thus, if a lake or stream is acidic, or the soil is acidic, shells, and therefore the animals that make them to protect and support themselves, cannot survive. Also, below a certain depth in the ocean (which varies with temperature and mineral content), calcium carbonate cannot be deposited, since the water is under-saturated with Ca CO3. This is called the calcium carbonate compensation depth , and no shell-bearing molluscs can survive below this level. To summarize, shells can only be formed in fresh waters that are non- acidic, and in the ocean at depths above the level where the water becomes under saturated with Calcium Carbonate. Shell Structure A typical gastropod shell is composed of three layers: The outer periostracum The middle prismatic layer (Ostracum) The inner nacerous layer (Hypostracum). The periostracum is thin and composed of a horny organic material called conchiolin, which is semi-transparent, being a brown color. The two inner layers are composed of calcium carbonate. In the middle layer, the calcium carbonate is laid down as vertical crystals. 19 In the thin, inner, nacerous layer, the calcium carbonate is laid down in thin horizontal sheet. The periostracum and prismatic layer are secreted by a marginal band of cells, so that the shell grows at its outer edge. Conversely, the nacreous layer is derived from the main surface of the mantle. Some shells contain pigments which include compounds such as pyrroles and porphyrins. Classes of Phylum Mollusca Monoplacophora Neopilina galatheae polyplacophora Chitons Gastropoda Cowries, Limpets, Slugs and Snails Scaphopoda Tusk Shells Bivalvia Bivalves = Muscles, Clams etc. Cephalopoda Nautilus, Octopus and Squid 20  Class Monoplacophora bearing a single single plate (shell) Class Monoplacophora Ex: (Neopilina) They are known by their fossil records. They live at depths of 200 to 6,000 meters and they range in size from 2 to 35 millimeters Monoplacophorans possess a single, large, bilateral shell. The shell is a simple depressed limpet or disk -shaped and is often thin and fragile. The outer surface of the adult is covered with a protective horny periostracum, or sheath. The nacreous layer of monoplacophoran shells appears to have undergone some modification where it consists of "brick-like" crystals of aragonite.These bricks are more like layered sheets( foliated aragonite). Class Polyplacophora bearing several plates (shell) The Polyplacophora contain about 900 living species and are commonly known as chitons. All chitons are marine inhabitants and most make a living by grazing algae from rocks and other hard substrates. The great majority of them dwell in shallow and intertidal waters, but a few occur in depths down to 5,000+ meters. They range in length from 3 to 400+ millimeters. They have a shell consisting of eight, usually overlapping calcareous plates, held together by a leathery "girdle". 21  )Acanthochiton fascicularis( )Acanthopleura haddoni( 6mm Class :Scaphopoda Commonly known as "tusk shells " Ex: Dentallium & Cadulas comprise about 350 livingspecies. The name "tusk shell" is derived from the fact that it resembles the shape of an elephant's tusk. The shell is usually an elongated, cylindrical tube, open at both ends, and slightly curved. The tusk or tooth shells, as they are more commonly known, have the simplest shell structure and anatomy of all the molluscs. They show very little variation in structure except that the members of the Dentalium resemble elephant tusks, while those of the Cadulus look like swollen cucumbers open at each end. 22  The shell is usually heavily ribbed and has small slits at the narrowest end. Colours range from white & off-white, cream, brown to an attractive variety of greens. The shells of most species average between one to two inches (2.5 cm - 5 cm) in length; ranging from 4 mm (Cadulus mayori) to a maximum of about 150mm (6") (Dentalium vernedei Sowerby, The western tribes of North American Indians once used tusk shells extensively, first as necklaces and later as money belts (Wampum) to be traded with other tribes, and the white man. All scaphopoda are marine, and live buried in muddy or sandy bottoms. Class : Cephalopoda This class includes octopus, squid, cuttlefish and nautilus. Octopus Squid Cuttlefish All are marine inhabitants and are predators or scavengers. Only a few cephalopoda produce a calcareous shell. Nautiluses are the only extant cephalopods which have an external shell. Cuttlefish, , Spirula, squid have small internal shells. Nautilus Nautiluses are the sole living cephalopods whose bony body structure is externalized as a shell. The animal can withdraw completely into its shell and close the opening with a leathery hood formed from two specially folded tentacles. The shell is coiled, aragonitic, nacreous and pressure resistant, imploding at a depth of about 800 metres (2,600 ft). 23  The nautilus shell is composed of 2 layers: a matte white outer layer, and a striking white iridescent inner layer. The innermost portion of the shell is a pearlescent blue-gray. The osmena pearl, contrarily to its name, is not a pearl, but a jewelry product derived from this part of the shell. Internally, the shell divides into camerae (chambers), the chambered section being called the phragmocone. The divisions are defined by septa, each of which is pierced in the middle by a duct, the siphuncle. As the nautilus matures it creates new, larger camerae, and moves its growing body into the larger space, sealing the vacated chamber with a new septum. The camerae increase in number from around four at the moment of hatching to thirty or more in adults. The shell coloration also keeps the animal cryptic in the water. HOW? When seen from above, the shell is darker in color and marked with irregular stripes, which helps it blend into the dark water below. The underside is almost 24  completely white, making the animal indistinguishable from brighter waters near the surface. This mode of camouflage is named countershading. The nautilus shell presents one of the finest natural examples of a logarithmic spiral. Nautilus hemishell Cuttlefish (Sepia) Cuttlefish (Sepia) possess an internal structure called the cuttlebone (or cuttlefish bone). The cuttlebone, which is a chambered and gas-filled shell, is unique to cuttlefish and one of the features contrasting them with their squid relatives. The cuttlebone is porous, composed primarily of calcium carbonate, and with a structure consisting of narrow layers connected by numerous upright pillars. The cuttlebone provides the cuttlefish with buoyancy control, with buoyancy regulated by changing the gas-to-liquid ratio in the chambered cuttlebone. Each species of cuttlefish has a distinct shape, size, and pattern of ridges or texture on the cuttlebone. In the past, cuttlebones were used in making polishing powder. The powder was added to toothpaste and used as an antacid or as an absorbent. Today, cuttlebones are commonly used as calcium-rich dietary supplements for caged birds and turtles. In addition, because the cuttlebone is able to withstand high temperatures and also be easily carved, it makes for an ideal mold-making material for small metal casting. 25 Spirula spirula S. spirula has a squid-like body between 35 mm and 45 mm long. End of mantle showing the photophore which emits green light. Live specimens of this cephalopod are very rarely seen, because it is a deep- ocean dweller. It very commonly floats ashore onto tropical beaches (and sometimes even temperate beaches) all over the world. Shell There is an internal calcareous shell.This seashell is known to shell collectors as the ram's horn shell or simply as Spirula. The shell of Spirula is extremely light in weight ,very buoyant and in the shape of an open planispiral (a flat spiral wherein the coils do not touch each other), and consisting of two prismatic layers. It is divided into approximately 25 to 37 chambers connected by a siphuncle. ; the gas-filled chambers keep the spirula in a vertical, head-down attitude. This shell serves as a hydrostatic system, allowing the animal to control its buoyancy. The shell is located in the posterior half of the mantle, and its buoyancy pattern results in a characteristic "head down" positioning often observed in Spirula. Squid (Loligo) The ancestral shell of squid (Loligo) has been lost, with only an internal gladius, or pen, remaining. The shell is a flexible rod, called a gladius. The pen is a feather-shaped internal structure that supports the squid's mantle and serves as a site for muscle attachment. It is made of a chitin-like substance. 26 Genus Argonauta (octopus) Females of the octopus genus Argonauta secrete a specialised paper-thin eggcase in which they partially reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal. Argonauta Linnaeus 1758 paper nautilus Argonauts are muscular, pelagic octopods. Females secrete a thin calcareous "shell" in which they reside. Shells may reach a length of 30 cm. The dorsal arms of females are modified with large, flag-like membranes that expand over the shell and are responsible for the secretion of the shell. Class: Gastropoda Gastropods generally have a single-valved shell. Starting at the apex, the smallest and oldest part of the shell whorls are coiled about a central axis called the columella. First shell whorls laid down by the larval gastropod (i.e., while it is in its egg), are called the protoconch (proto = before, conch is shell). It is represented by the smallest few whorls at the apex of the shell, and is usually smooth, and lacks many 27  of the characteristics of the adult shell, often being colorless, or of a different color from the rest of the shell. o umbilicus of a shell is the axially aligned, hollow cone-shaped space within the whorls of a coiled mollusc shell. The term umbilicus is often used in descriptions of gastropod shells, i.e. it is a feature present on the ventral (or under) side of many (but not all) snail shells, including some species of sea snails, land snails, and freshwater snails. The majority of the tens of thousands of marine and non- marine gastropod species are normally dextral (right handed - i. e., the shell opening is to the right when held spire upwards) while only a limited number of taxa are normally sinistral (left handed - the shell opening is to the left when held spire upwards). However, accidents of nature do happen, and occasionally a sinistral specimen of a normally dextral species or a dextral specimen of a normally sinistral species will be found - albeit rarely. 28  The edge of the shell that surrounds the aperture is the lip (sometimes called the peristome). The morphology of the lip of the shell of an adult snail is an important taxonomic character. The lip could be the same thickness as the rest of the body whorl (below, left). In such cases, it is referred to as a simple lip. In many species, however, when the snail is nearing the end of its growth, the lip of its shell considerably thickens. Thickened lips usually flare out and are referred to as being reflected. Many species of snails thicken the lips of their shells as they mature reproductively. Once their lips thicken, such species stop growing further and are said to have determinate growth. In contrast, snails whose shells retain a simple lip after they mature may continue to grow as long as they live, although their growth rate seems to slow down as they get older. They are said to have indeterminate growth. It is difficult to tell when a snail with indeterminate growth becomes sexually mature without dissecting it. Some shells have white rib forms inside the body whorl immediately behind the lip. Shells of many gastropods bears either a horny periostracum or calcium carbonate disc, called the operculum. This structure is found on the posterior dorsal (the back bottom) portion of the foot. This is the operculum, and it acts as a trap door that the animal can pull shut to close off the mouth of its shell, thus protecting its soft body parts, which are safely inside. The operculum may also be closed tight to guard against dehydration, if it should become necessary. (As in during dry periods or 29 winter (which in many parts of the world is just a dry season), or when a pond dries up!) **Some Families of Gastropods** In some gastropods, the shell is only conspicuously coiled in the juvenile stages. The coiled nature disappears with growth, and the adult shell represents a single large expanded whorl. Examples of this are found amongst the abalones (Haliotis), limpets (several families, including Lottiidae and Acmaeidae) and slipper shells (the familiar Crepidula and Capulus). Capulus (Capulus) Collisella pallidula Crepidula fornicata Haliotis rufescens badius Pectinodonta rhyssa(Linnaeus, 1758) (Swainson, 1822) (Gould, A.A., 1859) (Dunker, R.W., (Dall, W.H., 1919)Calyptraeidae Common Haliotidae Lottiidae 1882) Acmaeidae Atlantic slippersnail Red Abalone Snowy limpet Capulidae Limpet The word Limpet is a common name for a number of different kinds of saltwater and freshwater snails (aquatic gastropod molluscs). It is applied to those snails that have a simple shell ,the Chinese hat looking which is basically conical in shape, and either is not coiled, or appears not to be coiled in the adult snails. The name is given on the basis of a limpet-like or "patelliform" shell. The true limpets are all marine. Patellidae Fissurellidae Haliotidae Family : Patellidae Common name "true limpets". Patella vulgata Aquatic snail with a shell that is broadly conical in shape. 30 This general category of conical shell is technically known as "patelliform", meaning dish-shaped. Ex :Patella Linnaeus, 1758 Family : Fissurellidae Common name ( keyhole impets ) Keyhole limpets somewhat resemble true limpets because of the simple conical shape of their shells, but in reality they are not closely related to true limpets This conical shape of the shell allows keyhole limpets to withstand wave attack on exposed rocks. The shell has a reticulate (= net-like) sculpture with strong radial ribs lacks an operculum. For respiration, the shells of fissurellids have a single apical or subapical perforation ("keyhole"). This opening at the top allows a direct exit of exhalant water currents together with waste products from the mantle cavity. Ex : Fissurella Family: Haliotidae Common name (Abalones) Abalones are primitive molluscs bearing a shell that is rounded or oval with a large dome towards one end. The shell has a row of marginal respiratory pores and a wide aperture pressed close to the ground. The thick inner layer of the shell is composed of nacre, which in many species of abalone is highly iridescent, giving rise to a range of strong, changeable colors, which make the shells attractive to humans as decorative objects, in jewelry, and as a source of colorful mother-of-pearl. Ex: Haliotis 31 Family: Pleurotomariidae Common name (Slit shells) the slit shells consists of top shaped shells characterized by a remarkable slit in the edge of the outer whorl. A circular operculum is present, and the slit permits discharge of the excretory stream. Sculpuring of the shell is not deep, and the reddish beading follows spiral lines. Entemnotrochus rumphii Perotrochus Mikadotrochus salmianus Bayerotrochus westralis (Schepman, M.M., 1879) hirasei (Rolle, 1899) (Whitehead, T., 1987) 137 mm (Pilsbry,1903) 78 mm 120 mm Gastropods.com Gastropods.com Gastropods.com Ex: Perotrochus Family : Janthinidae Common name( purple snails or violet snails) This is a small family of snails that float on the surface of the sea, supported by a bubble float made by the animal. There are only two living genera in the family. The genus Janthina, the violet snails, has only five species, all with fragile, purple coloured shells. The genus Recluzia is poorly know but has only a few species, with fragile, brown shells. Ex: Janthina 32 Family : Epitoniidae common name ( Wentletrap) The word wentletrap originated in Dutch (wenteltrap), and it means spiral staircase. Common name "staircase shells", and "ladder shells". Wentletraps are small, often white, very high-spired, predatory or ectoparasitic sea snails, marine gastropod mollusks, in the family Epitoniidae. Wentletraps are notable for their intricately geometric shell architecture. The more or less turret-shaped shell consists of tightly- wound (sometimes loosely coiled), convex whorls which create a high, conical spiral. Spiral ribs (also called "striae") often occur. These shells often contain an umbilicus. The shells have a roundish or oval aperture, but its inner lip is often reduced to strip of callus. The round and horny operculum is paucispiral and fits the aperture tightly. Most species are small to minute with length between 0.6 and 11.7 cm. Within the genus Epitonium the shell has high, sharply ribbed sculpture with predominantly axial sculpture, known as "costae". These offer some protection against other predatory snails who find it hard to bore a hole in such a shell. Most species of wentletrap are white, and have a porcelain-like appearance. These shells are prized by collectors. Ex: Epitonium Family : Cypraeidae Common name ( cowries) These are marine gastropod molluscs. Long prized by primitive people for their shape and attractive colors, they were widely used for money in ancient times. 33 Shell description Cypraeidae have very rounded adult shells, almost like an egg. The shells are extremely smooth and shiny. This is because in the living animal, the shell is nearly always fully covered with the mantle. Typically, no spire is visible in the fully adult shell. There is a long, narrow, aperture which is lined with "teeth." Juvenile cowry shells are not at all similar to adult cowry shells. The juvenile shells of cowries perhaps more closely resemble the shells of some "bubble snails" in the order Cephalaspidea. Also the shells of juvenile cowries seldom exhibit the same color patterns as the adult shells do, and thus can be hard to identify to species. Cowries have no operculum. Predators and prey The very narrow toothed aperture of the cowry shell makes the adult shells difficult for many predators to reach into. However cowries are still vulnerable to predation: Some tropical crustaceans can break the dorsum of a cowry shell. Some mollusc-eating cones, such as Conus textile, can inject venom into the cowry's flesh. The cone then extends its stomach into the shell, through the aperature, to completely ingest the flesh. Some octopuses can gouge a small hole (using a special barb/tooth and an acidic secretion) through the shell to inject a venom that kills the animal within. Ex: Cypraea Family : Strombidae Common name (true conchs) Lambis lambis Strombus Linnaeus, C., 1758 34 Strombidae, is a taxonomic family of medium-sized to very large sea snails, marine gastropod mollusks. The family Strombidae includes the genera Strombus, Lambis, Tibia, and their allies. The term true conchs, being a common name, does not have an exact meaning. It may refer generally to any of the Strombidae but sometimes is used more specifically to include only Strombus and Lambis or just Strombus itself. Strombids have a long and narrow aperture, and a siphonal canal. The shell margin has an indentation near the anterior end which accommodates one of the eye stalks. This indentation is called a strombid or stromboid notch. The shell of most species in this family grow a flared lip upon reaching sexual maturity, and they lay eggs in long, gelatinous strands. Strombid were widely accepted as carnivores by several authors in the 19th century. Nowadays, strombids are known to be specialized herbivores and occasional detritivores. Unlike most snails, which glide slowly across the substrate on their foot, strombid gastropods have a characteristic mean of locomotion, using their pointed, sickle- shaped, horny operculum to propel themselves forward in a so-called leaping motion. Burrowing behavior, in which an individual sinks itself entirely or partially into the substrate, is also frequent among strombid gastropods. Ex: Lambis & Strombus Family: Conidae Common name (cone snails) The family Conidae is currently dominated by a one genus, Conus, which has about 500 living species. The shells of cone snails are shaped roughly like an ice- cream cone. The narrow end of the cone shell is the anterior end, and the wide end shows the usually very low spire of the gastropod shell. The aperture is elongated and narrow. The operculum is very small. The outer lip is sharp and without a callus. 35  The shells of cone snails are often brightly colored, and have interesting patterns, although in some species when the snail is still alive, the color patterns may be partially or completely hidden under an opaque layer of periostracum( a transparent yellowish or brownish membrane). The shell may be smooth or spirally ornamented, and the patterns and colors are extraordinarily varied. The bright colors and patterns of cone snails are visually striking, which often leads people to pick them up and hold them. However, this can be risky, as the snail may fire its harpoon in these situations. In larger species of cone snails, the harpoon can even penetrate skin, sometimes piercing through gloves. These snails hunt and immobilize prey using a modified radular tooth(called a toxoglossan radula) along with a poison gland containing neurotoxins; this is launched out of its mouth in a harpoon-like action. Some Cone snails are molluscivorous species , Conus aulicus, C. marmoreus, C. omaria, C. pennaceus or C. textile The venom of a few larger cone species is powerful enough to kill a human being. Fatal envenomation was evidenced by Conus aulicus, C. geographus, C. striatus, C. textile a C. tulipa. There is no antivenom. 36  Family: Vermetidae Common name ( worm shells) In the family of Vermetidae (the worm shells), the larval and juvenile shells are typical, but as the animal grows older the whorls become completely separate. The adults look much like a corkscrew, and sometimes don't coil at all, forming a tangled mass of tubes called a colony. The Worm shells are totally immobile and are either attached to other molluscs or entangled in sponges. Micromolluscs There is a largely unseen group of shells on the world's beaches. They are the micromollusks, shells which have all the beauty of their larger counterparts, but which remain unseen due to their small size. A micromollusk is a shelled mollusc which is extremely small, even at full adult size. Micromollusc can be seen by using a simple magnifying glass or a microscope. Considerable numbers of marine gastropod species are only about 5 or 6 mm in adult size; many others are only about 2 or 3 mm in adult size; and a few have adult shells which are as small as one millimeter or even smaller still. The maximum size of the shell of a micromollusc species is usually 5 mm to 7 mm.Micromolluscs are not very popular as a subject of study, even among professional malacologists, primarily because these minute species can be very challenging to work with. Discriminating the features necessary for successful identification of micromolluscs to the species level almost always requires a stereo or dissecting microscope. Identifying, or adequately photographing, the smallest species may sometimes require a scanning electron microscope. Family : Caecidae The shells in this family, are unusual in that the adult shell is a curving tube. The protoconch is lost and the tube is sealed with a permanent calcareous plug operculum at one end. Ex: Caecum 37 Family : Eulimidae The Eulimidae is a family of molluscs that is parasitic on echinoderms – starfish , sea urchins …. This family is of world wide distribution. The majority of shells are less than 10 mm in length, but a few reach up to 25 mm. In the descriptions terms are used relatively ; small means less than 4mm in length , large means large than 10mm in length. Shell colour is often plain white , or colourless transparent in minute species , but a few of the large species are brown and patterned. A feature of the family is that the spire is often bent, which is a useful character for species identification. Shell color is a useful characteristic for identifying species, as minute species start as colorless and transparent but become opaque white with age. In most species the suture is indistinct, but in transparent or translucent shells the suture of the internal surface of the shell is clearly visible. In the descriptions references to the suture always refer to the true, external suture. Class: Bivalvia Clams, cockles, mussels, oysters, scallops, and shipworms are bivalves. 38 The shell of a bivalve is two, usually similar parts called valves. Most are completely enclosed by the shell, the two valves of which are joined by an elastic ligament, and by two sheets of tissue called the mantle. The ligament that, usually in conjunction with interlocking "teeth" on each of the valves, forms the hinge. “Pedal muscles”. The foot is attached to the shell by a number of “pedal protrac- tor and retractor muscles”. One of the most important features of the bivalve shell is the “pallial musculature” that is associated with the attachment of the mantle to the shell. The mantle tissue, which secretes the shell, is attached by a series of muscles close to the margins of the shell. “Pallial line” a linear scar known running close to the edge of the shell. “Pallial sinus” a posterior indentation of the pallial line reflecting the development of the siphons. Muscle Scars Bivalves use muscles to close their shells. These scars indicate the position of the muscles in the bivalve's body.“Adductor muscles” typically two, situated medially close to the anterior and posterior margins serve to close the valves. “Anterior adductor and posterior adductor scars” impressed scars in the shell typically circular to oval, rarely elongate or crescentric in outline. 39  Type of muscle scar “Dimyarian” where both “Heteromyarian” where “Monomyarian” where adductors adductors are unequal in size.The only a single adductor is pres are present. anterior portion of the shell is the condition is often reduced, resulting in the termed “Homomyarian” where diminution of the anterior adductors are of equal size. adductor. Hinge A bivalve's two valves are connected by a hinge with a series of interlocking teeth and sockets that keep the valves aligned, especially when opening and closing. Hinge Ligament The ligament joins the valves along the hinge margin, it is elastic, mostly brown or black in colour and serves to open the shell when the adductor muscles relax. It is an important diagnostic character but is a complex structure and often difficult to interpret. Position The ligament nearly always lies between or posterior to the beaks. It may be easily visible when the valves are closed “external” partly visible “sunken” or not visible “internal”. OrientationThe ligament may lie symmetrically between the beaks “amphidetic” or more often behind the beaks “opisthodetic” and very occasionally in front of the beaks “prosodetic” Structural types 40 There are four main types. “Alivincular” is a flattened, usually triangular, structure lying between the beaks on a flat cardinal area and composed of a central fibrous layer bounded by lamellar layers on either side. “Duplivincular” is where there are multiple bands of fibrous and lamellar ligament inserted on the cardinal area, usually forming v-shaped chevrons. “Parivincular” is an arched structure lying behind the beaks. “Planivincular” an elongated weakly arched ligament extending posteriorly as a narrow band. 41 Type of bivalve dentation In bivalves, the term dentition is used to describe the arrangement of teeth, sockets and other closely related features. There are three types of dentitions present in the bivalves: Taxodont, Schizodont and Heterodont. a) Taxodont: it is a primitive type of dentition, where teeth are numerous, more or less equal in size and arranged in a subparallel pattern. b) Schizodont: it includes two or three thick teeth with prominent grooves. c) Heterodont: it comprises both cardinal and lateral teeth, which are not uniform, and less in number. The bivalve shells lacking dentitions are termed as Edentulous shells. 42