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. Linneus. 
A. rhomboidella. Plate 2. Fig. 52. 
Description. Shell rhomboidal, very inequilateral, com- 
pressed at base, longitudinally and closely ribbed ; sub- 
