50 



POTAMIDES concavus. 

 TAB. CCCXXXIX.— Figs. 1 aiid 2. 



Spec. Char. Subulato-conical, transversely striated ; 

 volutions rather concave towards the upper part, 

 costated ; costas obscure, arched, with a slight emi- 

 nence above the middle of each ; base with one of 

 two granulated carinae, convex ; lip enlarged below, 

 with a small groove in its upper angle. 



J- HE concavity near the upper edges of the volutions 

 crossed by the irregular costse is the distinguishing mark 

 of this shell : the columella is smooth with a produced 

 and somewhat recurved base ; the aperture is nearly 

 round, notched at the base, which forms a short beak ; 

 the transverse striai are not very deep, so that in worn 

 individuals they are wanting. 



I first received this from the Rev. Mr. Iremonger, 

 who collected it on Barton Clilf, see fig. 1 ; it has since 

 been met with in what is called the upper marine forma- 

 tion at Headon Hill on the Isle of Wight, by ray son 

 G. B. Sowerby ; a specimen brought by him is given at 

 fig. 2. This species differs from small worn specimens 

 of P. Melanioides, with which it might easily be con- 

 founded, in the length and curvature of its beak, as well 

 as the concave surface of its whorls. Fig. 3 represents a 

 small variety of P. Melanioides from Plumstead Heath 

 for comparison. 



