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LITHOPHAGA.— Three Genera. 



Boring shells, xoithout accessary pieces or sheath, and more or less gaping 

 at their anterior side. Ligament of the valves external. 



Saxicava. Shell bivalve, transverse, inequilateral; gaping anteriorly at 

 the superior margin; hinge almost without teeth. Ligament external. 



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



' gallicana, 



Petricola. Shell bivalve, subtriangular, transverse, inequilateral; the 

 posterior side rounded, the anterior attenuated; slightly gaping. Hinge 

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



P. lamellosa, P. striata, P. exilis, P. pholadiformis, 



' ochroleuca, ' costellata, ' ruperella, ' labagella, 



' semilamellata, ' rocelaria, ' chamoides, ' linguatula. 

 ' lucinalis, 



Venerirupis. Shell transverse, inequilateral; the posterior side very 

 short, the anterior gaping slightly. Hinge with two teeth in the right valve, 

 and three in the left; sometimes three in each: the teeth are small, approx- 

 imate, parallel, and but little or not at all diverging. Ligament external. 



V. perforans, V. irus, V. distans, V. carditoides. 



' nucleus, ' exotica, ' crenata, 



NYMPHACEA.— Ten Genera. 



Tico cardinal teeth, or more, on the same valve; shell often gaping slightly 

 at the sides. Ligament external; nymphaz in general, projecting outside. 



This family is divided into N. Solenaria and N. Tellinaria. 



NYMPHACEA SOLENAPJA. 



Sanguinolaria. Shell transverse, rather elliptical, gaping slightly at the 

 lateral extremity; the superior margin arched, not parallel to the inferior. 

 Hinge with two approximate teeth in each valve. 



S. occidens, S. rosea, S. livida, S. rugosa. 



Psammobia. Shell transverse, elliptical or oblong, oval, rather flat, gaping 



