318 IMMUNE SHELLS 



Preadth nineteen-twentieths of an inch. 

 Inhabits the southern coast. 

 Cabinet of the Academy. 



Occurs sometimes, cast on shore generally in frag- 

 ments, but is by no means a common shell. 



SAXICAVA, Bellevue. 



Species. 



S. *disforta. 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 promi- 

 nent ridge passing from its inferior angle to the beak. 



Length about three-fifths of an inch. 



Width about one inch. 



Inhabits the southern coast. 



Cabinet of the Academy and Philadelphia Mu- 

 seum. 



When young, it is generally more or less contract- 

 ed near the middle of the basal margin, but this 

 character decreases as the shell increases iu size, un- 

 til 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 resem- 

 bles Mytillus rugosus of Liu. but appears to be a 

 much thicker shell. It is generally imbedded in our 

 large Tliethya, Lam. and not unfrequently intervenes 



