GENERA OF SHELLS. 45 



marginal, composed of sulciform, transverse, 

 parallel teeth, not opposed, between which the 

 ligament is inserted ; a sinus for the byssus a 

 httle gaping under the end of the hinge ; 

 parietes callous. 



The hinge of the perna differs from that of the area 

 by the cardinal teeth of one valve not articulating v^^ith 

 those of the other, but lying upon them when the valve 

 is shut. The ligament is also differently situated. They 

 more resemble crenatjLilae. They are sea shells, w^ith 

 small, nearly equal beaks, situated at one of the extre- 

 mities of the hinge. 



P. ephippium, obliqua, isogonum, avicularis, femoralis, 

 canina, marsupium, sulcata, vulsella, nucleus. 



Malleus. Shell subequivalve, rough, de- 

 formed, generally elongated, sublobate at base ; 

 beaks small, divergent. Hinge toothless ; an 

 elongated conic groove, under the hinge, trav- 

 ersing the area of the ligament obliquely ; lig- 

 ament subexternal, short, received into the 

 sloping area of each valve. 



The mallei are distinguished from the pernœ by 

 their hinge ; from the aviculae by the conical pit, below 

 the beaks, and by the valves being, though irregular, of 

 the same size, and having no sinus on the left valve. 

 They are coarse, irregular shells, remarkable for their 

 form, with little beauty externally, but are rather bril- 

 liant pearly internally. 



M. albus, vulgaris, normalis, vulsatellus, anatinus, decur- 

 talus. 



