202 CORALID.'E. 



which anteriorly bounds the rostrum by which the hinder 

 side is terminated. The fragility of its valves is excessive; 

 they are transparent white, and so peculiarly delicate in 

 structure as almost to be membranaceous. The epidermis 

 is slightly cinereous, or at most faintly ochraceous, present- 

 ing, however, scarcely any tinge of colouring ; and the sur- 

 face, which is rather glossy, and elsewhere nearly smooth, 

 exhibits anteriorly, in the fully developed specimens, some 

 obsolete narrow concentric plicro. The beaks are acute 

 and incurved, and the umbones sufficiently prominent to 

 disturb, by their projection, the otherwise ovate or obovate 

 contour. Behind them, a more or less raised, not parti- 

 cularly oblique line, succeeded by a marked concavity, 

 divides the shell into two portions, which widely difier 

 both in convexity and size ; the front being ventricose, 

 and occupying more than three-fourths of the entire area, 

 whilst the hinder one, which is small, triangular, and 

 comparatively compressed, forms a short and very obtuse 

 beak, the lower edge of which is retuse or incurved, the 

 upper convex, and the extremity very distinctly hiant. 

 The ventral margin is arcuated, and much ascending at 

 each extremity ; the dorsal slopes are consequently very 

 trifling, and deviate but little from the rectilinear, except 

 at their extremities, which are more or less convex. The 

 front side, which is very decidedly the larger, is irregularly 

 rounded at its margin. The lateral tooth is obsolete. 



Our British specimens rarely, if ever, exceed one-third of 

 an inch in length, and somewhat less in breadth : indeed, 

 they are generally of far smaller dimensions. As a native 

 of our Isles, its existence was first published by Mr. Jef- 

 freys, who stated in the " Annals of Natural History," for 

 May 1847, p. 314, that Mr. Barlee had taken from seventy 

 to eighty individuals in Loch Fyne, from which locality 



