TO GEOLOGY. 15 
stance of the shell thin ; beaks small, potnted ; ribs about 
thirty-three, obsoletely tuberculated on the anterior por- 
tion ; teeth lamellar, oblique ; cicatrices scarcely percepti- 
ble ; cavity of the shell rather shallow ; margin crenulate. 
Diam.... Length .2, Breadth 7-20ths, of an inch. 
Observations. This pretty little species forms, by its 
parallel sides, nearly a perfect rhomboid. It has some re- 
semblance to the centenaria of Say, which is described by 
him, as well as Mr Conrad, as being “subrhomboidal.” 
The figures, however, are both trapezoidal, and, therefore, 
differ in outline from the description. It may also be dis- 
tinguished by the ribs, the centenaria being striate. The 
figure of Mr Sowerby’s duplicata has a close similarity to 
our shell, but differs in having the “ribs sulcated along 
the middle.” 
The genus rca has been observed in England, as low 
down as the Carboniferous Limestone. In Sweden and 
Germany, several species have been observed as low as 
the Cretaceous and Oolitic Groups. M. Al. Brogniart 
describes a species (Pandoris), from the Calcareo-trap- 
pean formation of Vicentin*. M. Deshayes gives us fifty- 
four species—twenty-three of these are from the Paris 
basin. From our formations, Mr Say has described three 
species, all from the Tertiary of Maryland. Mr Conrad 
has also described three, and observed two others common 
* Memoire du Vicentin, &c. page 76, 
