CHORISTODON. 



441 



with a byssiferous fissure a little behind the middle of the 

 lower edge. The cardinal or primary teeth of the hinge are 

 bent up as if arising from the inner surface of the valves 

 under the beaks. The species of Petricola appear to be 

 most numerous in Western America ; they are also found in 

 the Pacific, the Mediterranean and Bed Seas, India, and 

 New Zealand. They form excavations in limestone rocks 

 and coral, and also bury in mud. 



Species of Petricola. 



amygdalina, Sow. 

 bipartita, Desk. 

 Californica, Conr. 

 Chinensis, Desk. 

 cognata, G. B. Adams. 

 concinna, Sow. 

 cultellus, Desk. 

 cylindracea, Desh. 

 dactylus, Sow. 

 denticulata, Sow. 

 discors, Soiv. 

 fornicata, Say. 

 gibba, Midd. 

 gracilis, Desh. 



hyalina, Desh. 

 linguatula, Desh. 

 lithopbaga, Belz. 

 mirabilis, Desh. 

 nivea, Chem. 

 pboladiformis, Lam. 

 rariflamma, Desh. 

 robusta, Soiv. 

 rugosa, Soiv. 

 semilamellata, Lam. 

 serrata, Desh. 

 subglobosa, Soiv. 

 tenuis, Sow. 

 ventricosa, Desh. 



Genus CHORISTODON, Jonas. 



Shell ovate, rugose, tubercular, ribbed behind, tumid ; 

 beaks anterior. Hinge composed of two primary teeth in 

 each valve, the upper in the right valve compressed and elon- 

 gated, and in the left valve triangular, oblique, and bifid ; 

 lateral teeth none; ligament short, external, in a sunken 

 groove. Anterior adductor scar oblong, hinder very large, 

 rounded ; palrial line with a deep, round sinus. 



