TO GEOLOGY. 35 



Observations. The alternation of larger and smaller ribs 

 seems to distinguish this species ; except from that of a 

 living one, the dentalis of Lamarck, which is not so robust 

 a shell, while the cardinal ribs are larger. Var. B. (fossil) 

 I have not seen, but " costis majoribus planulatus" renders 

 it different. It differs in the ribs entirely from the attenua- 

 tum (Say), and is a thicker shell. In some specimens the 

 alternations are not entirely regular. 



D. tunitum. Plate 1. Fig. 3. 



Description. Shell slightly curved, smooth, polished, 

 rather thin; posterior termination furnished with four 

 turret-like appendages ; aperture round. 



Observations. A single specimen only of this curious 

 species has come under my notice. The turreted form of 

 the posterior or smaller termination does not seem to have 

 been noticed in any of the ten species described by Sowerby, 

 nor in the twenty-one by Lamarck. The space between 

 the turrets is festoon like. This remarkable termination 

 eminently distinguishes it from all other species yet de- 

 scribed. 



The smaller figure is of the size of nature. 



In Great Britain fourteen species have been obtained 

 from the Lias to the Crag. Four are mentioned by Sow- 

 erby as being in the London Clay. M. Deshayes's tables 

 give thirty-four, of which thirteen are from the Paris basin, 



