32 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



cally called teeth, and, together with the number and position of the 

 muscular points of attachment, chiefly serve to fix the classification. 



All the Tropiopoda are aquatic, either marine or fluviatile ; they live 

 buried in the sand, or in cavities of rocks ; some move about entirely free, 

 and others are either fixed immediately upon different marine bodies, or 

 are attached by a byssus of fibrous or silken filaments. 



The cicatrices exhibited in the interior of the valves, indicating the 

 points of muscular attachment, are either two in number and lateral, or 

 one and subcentral, and have been selected for the division of this class 

 into two orders as follows : 



Bimusculosa. 



Unimusculosa. 



Order I. TROPIOPODA BIMUSCULOSA. 



Testa, musculorum impressionibus duabus, distantibus, lateralibus, in- 

 terne imbuta. 



This order includes all the Tropiopodous Mollusca, in which the ani- 

 mal is bimuscular, or attached to its shell by two adductor muscles, the 

 marks of which attachment are exhibited in the interior, at the lateral 

 extremities of each valve. 



They are distributed according to their natural affinities into thirteen 

 families, as follows : 



tubicola. conchacea. 



Pholadaria. Cardiacea. 



SOLENACEA. ArCACEA. 



Myaria. Trigonacea. 



Mactracea. Naiades. 



LlTHOPHAGA. CHAMACEA. 



Nymphacea. 





