POROMYA. 205 



scabrous, the asperities not being arranged in any deter- 

 minate figure. Where these have been abraded, the surface 

 appears of a pearly white, pricked over with but slightly- 

 indented wavy punctures. The valves are rather ventri- 

 cose ; and very inequilateral, and are internally of a nacreous 

 white. The ventral edge is merely convex, but rises a little 

 behind, where for a short space it is slightly incurved as it 

 approaches the extremity of the linear but almost obsolete 

 umbonal ridge, which runs very obliquely from the umbones 

 to the lower corner of the posterior side, behind which the 

 surface is slightly depressed. The hinder side is nearly 

 twice as long as the other, and appears very indistinctly 

 biangulated, its posterior edge being scarcely convex, and 

 its dorsal one but very little sloping, and though slightly 

 curved, not far removed from rectilinear. The anterior ex- 

 tremity is obtusely rounded, the downward curve of the 

 upper margin being subarcuated, and rather sudden. The 

 beaks are acute, prominent, and leaning forward, without 

 any lunule in front of them. 



A single living example of this curious shell was dredged 

 by Mr. Jeffreys off the Island of Skye, in fifty fathoms 

 water. It was first captured in a recent state by Professor 

 E. Forbes in the ^gean, where it occurs at various depths 

 between forty and one hundred and fifty fathoms ; chiefly 

 below ninety fathoms. As a fossil it occurs in the upper 

 tertiary beds of Belgium, and in the coralline crag of Eng- 

 land, where it was found by Mr. Searles Wood. 



