458 CYPRINIDiE. 



Astarte), seems rather considerable, and neither accompanied 

 by much convexity posteriorly, or very marked concavity 

 anteriorly. The curve of the ventral margin is not unfre- 

 quently a little depressed towards the middle, but arcuated 

 at each extremity. The umbones are not elevated or pro- 

 minent, and the beaks when not eroded seem tolerably 

 acute. The lunule is large, not very profoundly excavated, 

 but acutely defined at the edges ; the margins of the linear 

 much elongated lozenge appear similarly well defined. 

 None of our specimens exhibit a ligament. The second or 

 posterior primary tooth of the right valve is clearly distinct 

 (it is often rudimentary or obsolete in this genus) ; the 

 muscular impressions are by no means peculiarly large, but 

 rather the contrary ; and the lower internal margins arc 

 strongly crenated. 



The only British Astarte at all akin to creiricosiata, is 

 sulcata, to which it is also allied in being of similar 

 dimensions, no other ribbed sjiecies possessing likewise a 

 crenated rim ; from this, however, the posterior cessation 

 of its narrower and more crowdedly set costse, affords a 

 ready mark for discrimination. 



The average length of the finer specimens was an inch 

 and one-fifth, their breadth being but little inferior to that 

 measurement. 



Some single valves were dredged by Mr. M'Andrew on 

 the west coast of Zetland ; and one has since been obtained 

 (Ann. Nat. Hist. 1847, vol. xix. p. 814) by Mr. Jeffreys, 

 in forty fathoms off Croulin Island, the Isle of Skye, where 

 it has also been taken by ]\Ir. M' Andrew. 



