The living body that includes soft parts like moue digestive, circulatory, urinary, reproductive, respiratory organs

The living body, which includes soft parts like moue digestive, circulatory, urinary, reproductive, respiratory organs and locomotors system is enclosed in a bivalve shell which is compress” laterally The respiratory system consists of a pair of leaf like gills and locomotors system is a plough shaped foot.

The shell opens to expose foot interiorly for its movement and the siphons posterior to take f and oxygen from water.

The calcareous shell made up of two equal valves placed on the right and left sides of the body. The height of the valve is measured from the ventral to dorsal margin: the lengths from anterior to posterior margin while the thickness from the centre of the right to the centre of the left valve. The anterior and posterior sides of the valve must be ascertained to differentiate the left and the right valves. Certain characteristics features are described below:-

(i) The umbones are directed towards the anterior side.

(ii) The valves are posterior elongated.

(iii) Lunule is present at the anterior while escutcheon at the posterior side of the umbo.

(iv) Pallial sinus is posterior placed.

(v) Out of two adductor impression the larger one is posterior placed.

(vi) Where only one adductor impression is present it is always posterior placed.

Once the anterior and posterior side of the valve is known, it can easily be placed on the side of the shell, to which it belongs. The valves are joined together in the dorsal margin along the hinge line and open at their ventral margin.

Dorsal side of each valve above the hinge line are drawn into a beak like structure, known as umbo (pi – umbones), which are commonly directed interiorly though in a few cases the umbones are posteriorly directed. The valves are held together by means of teeth and sockets present on the hinge line of each valve, which forms a ‘dental system’. The hinge line may be straight or curved and the number and nature of both teeth and sockets are variable in different forms.

The teeth and sockets of one valve fit into corresponding sockets and teeth of other valve. In a few cases a hinge area is found between the umbo and hinge line on both the valves. The hinge area of both the valves together form a small heart- shaped depression in front of the umbones, on the anterior side of the shell, known as Lunule while the other part the hinge area lies posterior to the umbones is bit elongated and is named as escutcheon.

The equivalved shell is in equilateral, as a line drawn from umbo to the central point of the ventral margin divides the valve into two unequal parts. Generally the valves are posterior elongated. The inner sides of both the valves are glossy to feel because of an enamel coating.

The valves are closed by means of one or two adductor muscles which run from interior of one valve to that of the other and the impressions of these muscles on the inner side of the valves are known as adductor impressions. In valves with two adductor impressions, the posterior adductor impression is generally larger than the anterior one. Sometimes both the adductor impressions are almost equal in size.

In a few cases there is only one adductor impression, which is commonly placed at the center of the valve. The opening of the valves is controlled by a ‘C’-shaped spring like ligament, found externally at the hinge margin above the escutcheon. The adductor impressions on each valve are connected with each other by a line known as pallial line caused due to the attachment of the mantle membrane. The pallial line may be simple or entire.

In a few cases the pallial line may have a fold like sharp bend near the posterior adductor impression, known as pallial sinus. Basing on the nature of dental system, the Pelecypods are placed under different groups:-

(i) Taxodont – Hinge line straight with numerous small similar sized teeth and socket.

(ii) Hetrodont – Few teeth of different shape and size.

(iii) Isodont – Two or three strong and equal sized teeth present in each valve

(iv) Schizodont – Very few, thick, grooved and strong teeth appear to diverge from below the umbones.

(v) Dysodont – Teeth numerous, small and similar in shape and size, radiate from below the umbones.

(vi) Desmodont – True teeth and socket are absent.

The shell is decorated externally with different colors which are seldom preserved in fossils. However, the pearly layer is sometimes preserved on the inner surface of the valves. The shells are ornamented with both coarse and fine radiating ribs and concentric rings. Tubercles and spines are present in some forms. In some cases, the shell exhibit wing-like projections on either side of the umbones, which are commonly described as ‘ears’. The ventral margin of the valve may be smooth or it may be crenulated and dentate.

Sometimes it becomes difficult to differentiate the brachiopod and pelecypod shells from each other. Below is given the salient features of shells of both the groups.

Lamellibranchia is a group of invertebrate animals having wide geographical distribution and geological range. The earlier fossils are found preserved in rocks formed during Middle-Cambrian period and continued to grow in number during the entire Paleozoic era. The Palaeozoic forms became extinct at the end of the era. New forms appeared and multiplied during Mesozoic era, a few of which are still living today.