Heterodonta is a subclass of bivalve molluscs, including the clam

Clam

The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...

s and cockle

Cockle (bivalve)

Cockle is the common name for a group of small, edible, saltwater clams, marine bivalve molluscs in the family Cardiidae.Various species of cockles live in sandy sheltered beaches throughout the world....

s. They are distinguished by having the two halves of the shell be equally sized, and having a few cardinal teeth separated from a number of long lateral teeth. Their shells lack a nacre

Nacre

Nacre , also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up pearls. It is very strong, resilient, and iridescent....

ous layer, and the gills are lamellibranch in form. Most species have a siphon.

The subclass contains two living and four extinct orders:

• Order †Cycloconchidae
• Order †Hippuritoida (rudists)
• Order †Lyrodesmatidae
• Order Myoida
  Myoida
  Myoida is an order of bivalve molluscs. They are burrowing molluscs, with well-developed siphons. The shell is relatively soft, and lacks a nacreous layer...
  
   (soft-shell clams and allies)
• Order †Redoniidae
• Order Veneroida
  Veneroida
  The Veneroida or veneroids are an order of bivalve molluscs. They include some familiar forms such as saltwater clams and cockles, and a number of freshwater bivalves including zebra mussels....
  
  (cockles and allies)