1. Brachiopods and pelecypods
Brachiopods is a phylum that includes organisms having a shell consists of two unequal valves (inequivalves) but pelecypods is a class of phylum Mollusca that includes organisms having mostly two equal valves (equivalves).
Phylum Brachiopoda
Class Pelecypoda
Phylum Mollusca

2. Chemical composition of shell
The shell is composed of CaCO3 in pelecypods (calcareous), it is also calcareous in most brachiopods although some have chitnophosphatic shell.

3. - The two valves articulate along a hinge line, which carries teeth and sockets.
They Close and open by muscles.
In brachiopods the larger valves contains teeth and the smaller valve contains sockets.
In pelecypods each valves carries teeth and sockets.

4. The mantle edge may be modified into tubes or siphons that are able to draw water and food into the gills and expel it. Typically the two tubes are located posteriorly and are designated the incurrent (inhalent) and excurrent (exhalent) siphons.

5. Muscle Scars
When the scars of two adductor muscles occur the bivalve is termed dimayarian. If the two scars are about the same size, the bivalve is isomyarian, in contrast to the anisomyarian in which the anterior scar is distinctly smaller.
Some bivalves like oysters have one adductor muscle scar, these are termed monomyarian.

6. Shell
The earliest part of the valves is called beak and the rounded part adjacent to the beak is called umbo.
The shell is bilaterally symmetrical with the plane of symmetry passing between the valves in pelecypods but bisects each valve in brachiopods.
Line of symmetry

8. Brachiopods attached by pedicle Pelecypods attached by byssus
Byssus: is an organ composed of chitinous fibers, which attach the pelecypod shell to the substratum. Brachiopods attach themselves by pedicle.
Brachiopods attached by pedicle
Pelecypods attached by byssus

9. Oysters
Oysters are pelecypods with two unequal valves, the larger valve is the left valve and is cemented to the substrate. The smaller valve acts as a lid or operculum.
They have only one muscle scar (monomyarian).
- There are neither teeth nor sockets.
- The beak is strongly curved.

10. Oysters
During Cretaceous oysters formed banks reaching more than 5 meters in height and extending for several kilometers.
They are still living today and live in aggregates cemented together.
Ex. G. Ostrea, G. Exogyra.

11. Rudistids
Rudists have also two unequal valves. The larger valve is the right valve, it has a conical shape and is fixed to the substrate.
The smaller valve is the left one, which acts as a lid.
- They have pachydont dentition.
- They died out (extinct) at the end of Cretaceous.