313 Bulletin 5 43 



FOSSIL SHELLS. 137 



elevated folds, producing very profound undula- 

 tions on the edge of the shell ; the intermediate 

 fold is bifid ; the whole surface is marked by rather 

 gross concentric wrinkles ; inner margin dusky or 

 blackish, with a series of granules on one valve, 

 received into corresponding cavities in the oppo- 

 site valve. 



Length one inch and a fifth, breadth one inch. 



This species is very distinct from the P. plicata, 

 lyinn. of the West Indies, and from that of the 

 East Indies, hitherto confounded with the plicata, 

 but to which Laraark's name of gibbosa maj^ be 

 transferred. 



Of the known fossil species, it seems to approach 

 nearest to the angulata, Lam. but the folds are 

 neither squamous nor angulated. 



ARCA. 



I. A. arata. PI. x. fig. i. Shell transversely^ 

 oblong, subrhomboidal, with about twenty-six lon- 

 gitudinal ribs ; basal edge nearly parallel to the 

 hinge margin, which latter terminates anteriorly 

 in an angle. 



Description. Ribs somewhat flattened, as wide 

 Or rather wider than the intervening spaces ; 

 the whole surface concentrically wrinkled : um- 

 bo7ies not remarkably prominent : apices remote, 

 the intervening space rhomboidal, with continued 

 indented lines, arcuated under the apices : hinge 



vol,. IV.— DECEMBER, 1824. 



[J. A. N. S., Phila., ist Ser., Vol. IV, 1824.] 



