Molluscs include five classes of invertebrate animals. They live in marine, freshwater as well as on land.

A few aquatic forms are free swimming but majorities are bottom dwellers. Out of the five classes mentioned above, lamellibranches (pelecypods) and gastropods are two forms living in abundant number while cephalopods are represented by nautilus, octopus, and squid.

The term lamellibranchia has been derived from two words lamella = leaf + branchia = gills i.e. they have leaf-like gills. Due to the presence of plough- shaped foot (pelecy = plough + poda = foot), they are also known as pelecypoda. They started their life during Cambrian and many of them continue till present day. Most of them are bottom dwellers, either as borers or crawlers, some of them.

Rhynchonella:

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Shell triangular, hinge line curved, umbo small and curved, anterior margin produced into a tongue like projection, surface ornamented with strong ribs. Age – Upper Jurassic.

Terebratula:

Shell biconvex, elongated, egg shaped, and large ventral umbo with pedicle opening, curved hinge line, anterior margin of both the valves with two folds, ornamented with fine concentric growth lines. Age – Late Mesozoic to Late Tertiary.