Basis for a Pelecypod Taxonomy

Pelecypods, bivalve molluscs, also known as bivalvia, can be subdivided into orders and suborders on basis of shell structure,  hinge teeth, and type of gills.  Those included in the Prionodesmata have shells that are prismatic and nacreous. Mantle lobes are separate, siphons poorly developed, hinge teeth lacking or unspecialized. Those included in the Teleodesmata have shells that are porcelaneous and in part nacreous. Mantle lobes are generally connected, siphons well developed, hinge teeth specialized.

Gills, which help determine suborder, may be  protobranch, filibranch, eulamellibranch or septabranch, going from the simplest and most primitive to the most advanced and evolved.

Orders are determined largely on the basis of dentition, that is hinge teeth. Order with included suborders are:
Taxodonta: hinge teeth numerous, subequal, and small.  Includes the Nuculacea which have protobranch gills and the Arcacea which have fillibranch gills.
Schizodonta:  teeth large and distinct, diverging from beneath the beak. Includes the Trigonacea have have trigonal shells and are marine, and Cariniacea which have rounded to elongate shells and are mostly found in fresh water. Both are fillibranch.
Isodonta: teeth are in symmetrical pairs on each shell (or valve).  Includes the Spondylacea in which shells are unequal and spiny and Anomiacea in which shells are thin, the lower commonly perforate.
Dysodonta: hinge teeth weak or absent. Includes the Mytilacea and Pectinacea, slipper shells and scallops, which have fillibranch gills and Pinnacea which are long and thin shelled, Ostreacacea, oysters, Limacea which resemble scallops, and Dreissemsiacea which are similar to the mytilids. The latter four are all eulamellibranch.
Heterodonta: teeth well developed and specialized into cardinal and lateral. Gills eulamellibrach. Includes, on the basis of dentition, starting with the most primitive, Cypricardiacea, Lucinacea, and Cyrinacea.
Desmodonta: hinge teeth weak or absent. Includes are Solenacea, razor clams, Myacea, which gape posteriorly, Mactacea, which have prominent siphons, Adesmacea, which include wood and rock borers, and Ensiphonacea in which shells are degenerate and siphons are enclosed in a calcareous tube, all of which are eulamellibranch. Also include are the Poromyacea  the only ones to have septibranch gills.

Then there are the pachydonts (Pachydonta), which include the reef forming rudist clams from the Cretaceous.

This particular scheme continues to be useful in paleontology, in the study and placement of fossil forms. Here gill types can only be inferred and extrapolated from recent, living examples.

Newer classifications are based mostly on criteria from the animal itself which can only apply directly to living forms.

I know I should probably include pictures and/or illustrations. I'll try and remember that for future blogs that are more specific in nature.