106 



length of the shell ; within hluish-white ; umho destitute of the 

 slightest elevation ; anterior and posterior margins rounded; supe- 

 rior and inferior margins rectilinear, parallel. 



Length seven-twentieths of an inch. Breadth nineteen-twen- 

 tieths of an inch. Inhabits the southern coast. Cabinet of the 

 Academy. 



Occurs sometimes cast on shore, generally in fragments, but is 

 by no means a common shell. 



Saxicava distorta. — Shell thick, inequal, rugged, trans- 

 versely oblong-subovate ; epidermis pale-brownish, much wrinkled ; 

 umbo prominent, placed very far back ; posterior margin rounded, 

 generally very short; anterior margin often truncated, with a 

 prominent ridge passing from its inferior angle to the beak. 



Length about three-fifths of an inch. Width about one inch. 

 Inhabits the southfi-n coast. Cabinet of the Academy and Phila- 

 delphia Museum. 



When young, it is generally more or less contracted near the 

 middle of the basal margin, but this character decreases as the 

 shell increases in size, until it disappears entirely in the adult 

 state. It is in other respects variable in form and proportion, the 

 beaks are rarely placed so far back as to be parallel with the tip of 

 the posterior margin ; it much resembles Mi/tillus rugosus of Lin., 

 but appears to be a much thicker shell. It is generally imbedded 

 in our large Thethya, Lam., and not imfrequently intervenes 

 between the substance of the Tlicthya and the sides of a large 

 Ascidia, which also attaches itself to that animal. It is also 

 sometimes found in a species of spongia. Pinnotheres hi/ssomm 

 of this Journal, inhabits this shell. The young shell is furnished 

 with a prominent incrassated hinge tooth, which closes into a cor- 

 responding depresssion in the opposite valve ; but this tooth dis- 

 appears with age. 



It is referrible to the genus Pholeohm of Leach. 



Petricola fornicata. — Shell transversely elongated, posterior 

 side very short ; anterior side a little gaping ; hinge and basal mar- 

 gins subparallel ; valves longitudinally radiated with elevated lines, 

 which, anterior to that which terminates at the middle of the base, 

 arc alternately more or less prominent, filiform, and all posterior to 

 that line are fornicated costa ; concentric wrinkles numerous, more 

 remarkable on the anterior margin ; lunule ovate-acute, simply 



