animal. The palleal impression is also very distinct. — 

 These dorsal cicatrices were, no doubt, taken into account 

 by Merj, Mem. Acad. Rojale des Sc. for 1710, p. 409, 

 who saj^s that there are eight muscles attached to the in- 

 ner surface of the shell. 



REFERENCE TO THE PLATE. 



Exterior, dorB&l and interior views. 

 .PI. 51. 



UNTO APICULATUS. 



DESCRIPTION. 



Shell subquadrate, the diameters being subequal ; sum- 

 mifs not very prominent : /mige margin declining, and 

 with the anterior margin compressed and separated by an 

 obtuse angle : anterior margin generally reluse : basal 

 margin retuse before the middle : posterior margin round- 

 ed : disk with a costa from the summit descending to the 

 anterior basal angle, which is somewhat prominent and 

 rounded ; behind the costa is a much dilated groove, not 

 very deeply impressed ; the whole surface, without excep- 

 tion, is studded with small, subequal eminences, which are 

 more or less rounded or transverse and arranged more or 

 less symmetrically in lines, which before the costa, curve 

 towards the liciament and anterior marsjin, are ansrulated in 

 the groove and on the middle and thence are arquated to- 



PI. 52. 



