TO GEOLOGY. 69 



shell thick ; lunule very small, deeply impressed, cordate ; 

 beaks rather elevated, recurved ; ribs about twenty-seven, 

 on the anterior part furnished with somewhat distant tu- 

 bercles ; teeth transverse ; cicatrices impressed ; cavity of 

 the shell deep, oblique ; margin very largely crenulate. 

 Diam. 1.3, Length 1.4, Breadth 1.9, of an inch. 



Observations. This species is perhaps most remarkable 

 for its transverseness and its large knotted ribs. Those of 

 the posterior part are rounded. It most resembles the Sil- 

 limani, but is more transverse. 



F. Sillimani. Plate 2. Fig. 47. 



Description. Shell rather transverse, inflated, inequila- 

 teral, covered with longitudinal, elevated ribs ; substance 

 of the shell thick ; lunule very small, very transverse, and 

 very deeply impressed ; beaks rather elevated, recurved ; 

 ribs, about twenty-seven, sharp and imbricate behind 

 obtuse and tuberculate before ; teeth nearly transverse ; 

 cicatrices slightly impressed ; cavity of the shell deep ; 

 margin largely crenulate. 



Diam. 1, Length 1.1, Breadth 1.4, of an inch. 



Observations. In many of its characters this species 

 closely resembles the transversa. But being less trans- 

 verse, having the posterior ribs more elevated and some- 

 what imbricate, I am induced to separate it. In dedicating 

 it to my friend Professor Silliman, I do myself a peculiar 

 pleasure. His great acquirements and successful investi- 



