MACTRA. 355 



the anterior side being-, if either, the more produced. Both 

 extremities are somewhat attenuated, but whilst the front 

 is rounded, the hinder is angulated below, the anterior 

 dorsal slope being strongly arcuated, and the posterior one 

 far more rectilinear. The umbones, which are elevated, 

 very prominent, and oblique, curve forward at their acute 

 apices ; on either side of them there is a considerable flat- 

 tening of the sides of the shell, which are invariably 

 grooved with crowded narrow sulci. This depression is 

 more marked behind, where it is defined by an indistinct 

 umbonal ridge, than in front, where it is confined to the 

 beaks, and is not co-extensive with the sulci. The liga- 

 ment, cartilage, teeth, and inner margin are similar to those 

 of solida. 



The length of the largest specimen we have seen was 

 two inches, and its breadth an inch and three-quarters. It 

 is distinguished from M. solida by the great projection of 

 its umbones, its triangular contour, the arcuation of its 

 ventral margin, the profundity of its valves, and, in the 

 more typical examples, the production of its anterior side 

 and its concentric striae. 



The animal closely resembles that of the last species. 



This shell is stated by Montagu to be extremely common 

 on the shores of the Frith of Forth, — where we have dredged 

 it in seven fathoms (E. F.), — and was dredged by Turton 

 in the Irish Channel. It is occasionally taken at Tenby 

 (S. H.), and is met with also in Cork harbour and 

 Dublin bay (Humphreys and Jeff, cab.) ; Bangor bay, 

 Down (Patterson). It is stated by Mr. Couch to inhabit 

 the shores of Cornwall, and is a littoral species at Stronsa 

 in Orkney (Thomas). 



It occurs fossil in the glacial beds of the Clyde (Smith). 



