74 CONTRIBUTIONS 



genus Pulvinites of Defrance, is still further removed, the 

 series of teeth being directly at the apex of the margin, 

 which is there thickened. 



P. costatus. Plate 2. Fig. 51. 



Description. Shell cordate, somewhat inflated, longitu 

 dinal, minutely and longitudinally ribbed ; substance of 

 the shell thin ; beaks elevated, recurved ; cicatrices large ; 

 the anterior series of the teeth short, the posterior one 

 long, anterior one longer and narrower ; cavity of the 

 shell rather deep ; margin very minutely crenulate ; nacre 

 pearly under the epidermis. 



Diam. .1, Length .2, Breadth 3-20ths, of an inch. 



Observations. This interesting little species requires a 

 microscope to observe some of its characters. The speci 

 mens in the best state of preservation are greenish. The 

 ribs, in some instances, diverge, and are cut by the lines of 

 growth. 



FAMILY ARCACEA. 



GENUS ARCA. Lmncetw. 



A. rhomboidella. Plate 2. Fig. 52. 



Description. Shell rhomboidal, very inequilateral, com 

 pressed at base, longitudinally and closely ribbed ; sub- 



