I. CONCHIFERA. 



Body without articulations or transverse seg- 

 ments ; without distinct head or eyes ; and 

 always inclosed in a bivalve shell. 



f Dimiaj-'ice. 



With two muscles of adhesion at least ; and two in- 

 ternal muscular impressions, distinct and lateral, 

 in each valve. 



a. ConchcE. 

 The valves with lateral teeth on each side the hinge. 



] . Cyclas. The lateral teeth of one valve double. 

 Type. Cardium corneum, Montagu. 



b. Naiades. 



The valves with a single irregular tooth, simple or 

 cloven ; or under the ligament of the hinge is a 

 transverse elongated lamina, or a row of tubercles. 

 Beaks generally decorticated, and often eroded or 

 rugged. 



