Lamarck's Genera of Shells. 35 



of the pema has a row of linear, parallel, truncated teeth, ar- 

 ticulating M ith those of the opposite valve, the ligament being 

 inserted in the interstices of the corresponding teeth. 



The crenatulae are rare shells, generally thin, sometimes 

 aln.ost membranous, and brittle. 



Type. Cranotnla jnodiolaris *. 

 Shellsub-cuneiform,compressed,sub-membranaceous, reddish, 

 radiated with white ; nates below the base, separated by a sinus. 

 South American Seas. 7 Species. PI. I. Fig. 81. 



2. Pernat. 



Shell subequivalve, flattened, rather deformed ; texture la- 

 mellar. Hinge linear, marginal, composed of sulciform, trans- 

 verse, parallel non-entering teeth, between which the ligament 

 is inserted. A posterior sinus, slightly gaping, below the ex- 

 tremity of the hinge, for the passage of the byssus ; sides 

 callous. 



The hinge of the perna is so peculiar, that it is surprising 

 Linnaeus ^should have classed it with the ostrese; it does not 

 even belong to the family of ostracea. It differs from the area, 

 by the cardinal teeth of one valve not articulating with those of 

 the opposite valve, but, when the shell is shut, lying upon them. 

 The ligament also is differently situated from that of the area. 

 They have much more resemblance to the crenatulae ; they are 

 sea shells, with small, nearly equal beaks, situated at one of the 

 extremities of the hinge. The shell, though pretty solid, is 

 composed of ill-joined laminae, as is the case, with the other 

 malleacea. 



Type. Perna ephippium J. (Ostrea ephippium. Linn.) 



Shell compressed, on the upper part orbicular ; posterior side 

 longest; margin very acute. Indian Ocean. 10 recent species, 

 and 2 fossil. PI, I. Fig. 82. 



• Allied to modiola. Lamarck's second species. His type is C. aricularis, 

 t Pema, strictly, is a gammon of bacon, with the leg on. It was used to 



denote a kind of shell fish, (very probably our perna,) from its resemblance 

 to a pig's foot. (Plin. 32. sub. fine.) 



♦ From tiri, uirnn, anti iTTTro;, a horse— a saddle, 



D 2 



