18 GENERA OF SHELLS. 



LITHOPHAGA. 



Boring shells, ivithout accessory pieces or sheath, and more or 

 less gaping at their anterior side; ligament exterior. 



Saxicava. Shell bivalve, transverse, ine- 

 quilateral, gaping anteriorly and at the supe- 

 rior margin. Hinge nearly toothless ; ligament 

 exterior. 



Sometimes the hinge has two distant, scarcely denti- 

 form tuberosities. The shells are short and obtuse 

 posteriorly, anteriorly longer, flatter, and often trun- 

 cated. They are small or of moderate size. 



S. rugosa, gallicana, pholadis, australis, veneriformis, 

 distorta. 



Petricola. Shell bivalve, subtriangular, 

 transverse, inequilateral, rounded posteriorly ; 

 the anterior edge sharp, slightly gaping. Hinge 

 with two teeth in each valve, or in one only. 



P. lamellosa, ochroleuca, semilamellata, lucinalis, striata, 

 costellata, roccellaria, exilis, ruperella, chamoides, pholadi- 

 formis, fabagella, linguatula, forriicata. 



Venerupis. Shell transverse, inequilateral, 

 posterior side very short, anterior slightly gap- 

 ing. Hinge with two teeth on the right valve, 

 and three on the left, sometimes three on each ; 

 teeth small, approximate, parallel, slightly diver- 

 gent ; ligament exterior. 



The hinge of venerupis appears analogous to that of 

 venus, but it is distinguished by the cardinal teeth. 

 They are perforating shells, and differ from the petri- 

 colfe by having three cardinal teeth, at least on one 

 valve. 



