TO GEOLOGY. 75 



stance of the shell thin ; beaks small, pointed ; 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 &quot; subrhomboidal.&quot; 

 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 &quot;ribs sulcated along 

 the middle.&quot; 



The genus Jlrca 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. 



