The geological ranges of some important species are as follows

They are confined to mostly marine environment, hence are found to be present in rocks of marine-nature.

The geological ranges of some important species are as follows:

1. Products:

Carboniferous to permian.

2. Orthis:

Ordovician to Silurian.

3. Spirifer:

Silurian to permian.

4. Atrypa:

Ordovician to devonian.

5. Rhyochonella:

Jurassic.

6. Terebratola:

Eocene to pliocene.

7. Terebratella:

Triassic to present day.

The brachiopods reached the acme of their development during the ordovician, Silurian and devonian periods. Within the rocks of tertiary age, brachiopods occur only occasionally.

Pelecypoda (Lamellibranchs) :

Pelecypods belong to the phylum ‘Mollusca’. The lamellibranchs are oysters. All are marine, some live on land, others in water and many on both. The body is bilaterally symmetrical. The shell consists of two valves which are equivalves and are placed on the right and left side of the body. The valves are inequilateral, i.e., a perpendicular line drawn from the umbo to the opposite margin does not divide the valve into two equal halves.

Both the valves are hinged together at their dorsal margin by means of teeth, sockets and with ligaments. Each valve has an umbo and near it the hinge-line, which marks the dorsal region of the animal. The region where the valves separate most widely when the shell opens is the ventral region. The margin near the mouth is anterior.

Sometimes there is in front of the umbones an oval-shaped depressed area of smaller size appears like a groove, shared by both the valves. It is known as ‘lunule’. Behind the umbones, there is a similar somewhat larger area, known as ‘escutcheon’.

The hinge is formed by projections known as teeth alternating with depressions or grooves known as sockets. The teeth and sockets in a hinge line, alternate with each other in the two halves and teeth of one valve fits into the sockets of the other valve.

Depending on the nature, shape and size of the teeth and sockets, a few distinct types of dentition have so far been recognised. The dentitions are as follows:

(i)Taxodont:

Also known as etenodont. Here the teeth and sockets are more or less similar in form and size e.g., Nucula, area.

(ii) Dysodont:

Here the teeth radiate outwards form the umbo, e.g., Mytilus, Ostrea.

(iii) Isodont:

Here the hinge consists of two strong and slightly curved teeth, which occur in each valve and fits into the sockets of other valve, e.g., Spondylus.

(iv) Schizodont:

Sometimes a few thick and occasionally grooved teeth are developed and these may vary in shape and size.

(v) Heterodont:

Also known as telodont, where the teeth are not of uniform shape and size and are few in number.

(vi) Desmodont:

Here true teeth and a binge plate, but one or more laminae or ridge are developed at the hinge margin. Commonly one cardinal tooth in each valve, e.g., Pleuromya.

(vii) Asthenodont:

In the burrowing lamellibranchs, the hinge is made up of obsolete teeth and the dentition is described as asthenodont.

(viii) Edentulous:

Lamellibranchs with no teeth are said to possess edentulous shell. This is also known as palaeoconcha.

The surface of the shell may be smooth or may be ornamented with radiating ribs, concentric ribs and striations, tubercles and spines etc. The margins of the valves may be smooth or crenulated.

The interior of the valves are marked with the impressions of the muscles. In a living animal there are elastic ligaments which perform the function of divaricators in case of lamellibranchs.

Usually two adductor impressions occur in the interior of each valve. One of these two is placed anteriorly and the other posteriorly. Both the impressions are connected together by means of a linear depression called the pallial line, which runs more or less parallel to the ventral margin of the valve. Sometimes there is a notch on the pallial line known as the pallial sinus.

When two adductors impressions are found to occur the shell is called a ‘Dimyarian shell’. When there is only one impression the shell is said to be a monomyarian shell. When both the impressions are of equal size, it is known as Isomyarian shell.

When the ligament lies below the hinge-line, it is called a resilium. A process for lodging the resilium is called chondrophore. Where the ligament extends on either side of the umbo, it is called amphidetic ligament but when it is entirely behind the umbo, it is described as Opisthodetic ligament.

Byssus is a thread like process derived from anterior portion of foot and used to attach the shell to substratum.

The first formed embryonic shell is called the ‘Prodissoconch’. The shape of the pelecypod shells may be alate, rostrate, mytiliform, quadrate, produced etc.