The geological ranges of some important pelecypods are as follows

The earliest records of fossil lamellibranchs have been traced from the rocks of lower-ordovician age, and they reached their culmination during the quaternary age. The geological ranges of some important pelecypods are as follows:

(a) Area:

Jurrassic to Recent.

(b) Nucula:

Silurian to Recent.

(c) Pecten:

Carboniferous to Recent.

(d) Ostrea:

Triassic to Recent.

(e) Gryphaea:

Jurassic to Eocene.

(f) Inoceramous:

Jurassic to cretaceous.

(g) Trigonia:

Jurassic to Recent.

(h) Cardita:

Eocene to Recent.

(i) Mya:

Tertiary to Recent.

Gastropods:

The molluscs belonging to the class Gastropoda have representa­tives which inhabit on the dry land as well as others that live in fresh and salt waters. In the majority of the gastropods, a shell is secreted by the mantle. The gastropod-shells are made up of one valve only and therefore is said to be univalve.

The simplest form of the shell is an elliptical cone widely open at the base, but in the great majority of cases, the shell is a cone coiled into a helicoids spiral, open at one end and tapering to a point at the other. Each one of the individual coils is described as a whorl.

All the whorls except the last together form the spire of the shell. The tapering end of the spire is known as ‘apex” and it forms the posterior end of the shell. The part of the whorl farthest from the apex is called the base of the shell.

In a few genera, the whorls are separated, but as a rule they are in contact. The successive whorl” lie in mutual contact along a line called the ‘suture’.

The coiling of the shell is usually dextral, i.e., when the apex of the shell is pointed upwards, the aperture will be on the right hand side, i.e., (Clockwise). But in a few cases it is sinstral (anticlockwise) when the aperture occurs on the left hand side, e g., is ‘Physa’.

Sometimes the inner faces of the whorls are united into a solid pillar extending from the base to the apex and are known as collumella, e.g., is ‘Turritela’. But in other cases, instead of being united centrally into a collumella, the whorls are coiled around a central cavity, which extends from the base to the apex of the shell. The cavity opens at the base of the shell; called ‘umblicus’.

The last body whorl ends in an opening, which is known as aperture of the shell. The form of the aperture varies considerably in different genera. The margin of the aperture is called the ‘peristome’ consisting of an inner lip lying towards the inner side of the last whorl and an outer lip. There is a homey or calcareous plate which closes the aperture of the shell when the animal with­draws into it. That calcareous plate is known as the ‘Operculum’.

The form of the operculum varies considerably in different genera.

The angle subtended at the apex between two straight lines drawn tangentially to the last two whorls on two opposite sides of the shell, is known as apical or pleural or spiral angle.

The coiled shells of gastropods exhibit a variety of forms,, depending on the following factors:

(i) Arrangement of the whorls in one plane or in a helicoids spiral.

(ii) Spiral angle.

(iii) Size of the last whorl and whether it conceals the earlier whorls or not.

The important types are:

(a) Discoidal, e.g., Planorobis.

(b) Conical or Trochiform, e g., Trochus.

(c) Turbinate (i.e., resembling a sppinnig top), e.g., Turbo.

(d) Turreted (where spire is long and shape is that of an acute cone), e.g.. Turritella.

(e) Fusiform (where the last whorl repeats the shape of the spire in inverted position), e.g., Fusinous, Rimellarimosa.

(f) Cylindrical, e.g., Pupilla.

(g) Globular (last whorl forms the greater part of the external surface, the spire being very low), e.g , Natica.

(h) Convolute (hemi-ellipsoidal, no spiral coiling is recog­nizable), e.g., Cypraea.

(i) Auriform- here the spire is short and the aperture in­conspicuously large.

The surface of the shell is frequently ornamented with spines, knobs, ribs or striae. In some genera like Murex, rows of spines, o lamellar processes, extend across all the whorls from the apex to the base of the shell, forming what are termed as varies, these are- elevations.

The embryonic shell is often found at the apex of the shell and is called the protoconch.