OF THE UNITED STATES. 319 



between the substance of the thethya and the sides 

 of a large ascidia, which also attaches itself to that 

 animal. It is also sometimes found in a species of 

 spougia. Pinnotheres byssomia of this Journal, in- 

 habits this shell. The young shell is furnished with 

 a prominent iucrassated hinge tooth, which closes 

 into a corresponding depression in the opposite valve; 

 but this tooth disappears with age. 



It is referiible to the genus Pholeobia of Leach. 



PETRICOLA, Lam. 



Species. 



P. *fornicata. Shell transversely elongated, pos- 

 terior side very short ; anterior side a little gaping 5 

 hinge aud basal margins subparallel; valves longitu- 

 dinally radiated with elevated lines, which, anterior 

 to that which terminates at the middle of the base, 

 are alternately more or less prominent, filiform, and 

 all posterior to that line are fornicated costa ; con- 

 centric wrinkles numerous, more remarkable on the 

 anterior margin; lunule ovate-acute, simply sculp- 

 tured with the concentric wrinkles ; within radiated 

 with strongly indented lines, which, on the anterior 

 margin, are obsolete ; teeth two, rarely three, on 

 each valve, one of which is bifid at tip or grooved 

 on the inner side, and the other usually not promi- 

 nent above the margin. 



Length three fifths of an inch. 



Width one inch and seven-tenths. 



