l68 MARSHALL: ON A BRITISH SPECIES OF MYRINA. 



Mypina simpsoni, n. sp. 



The shell is transversely oblong, convex, and of a thin texture ; 

 sculpture, microscopic and close-set strige which radiate from the 

 beaks, and equally fine transverse striae, with coarse and irregular 

 lines of growth; in the centre of each valve are five or six longitudinal 

 obtuse riblets, which cross the shell from the upper to the lower 

 margin ; epidermis yellowish-brown, persistent, highly polished, with 

 a blistered appearance over the central riblets ; colour under the 

 epidermis pearl-grey ; margins nearly straight at the upper part, 

 ascending very slightly from the umbones, gently incurved in the 

 lower margin, rounded on the anterior side with a greater slope from 

 the beaks, and sloping from the back on the posterior side and evenly 

 rounded, though this part is broader in some specimens than in others, 

 while both sides are always deeper than the centre ; umbones very 

 gibbous and swollen ; beaks incurved, and placed very near the 

 anterior side; ligament internal, large and strong; inside pearl-grey, 

 iridescent, microscopically rayed ; hinge-line almost straight ; hinge- 

 plate narrow, thickened, slightly and very finely crenated on the 

 posterior side; there is a small and obtuse excrescence on the anterior 

 side just below the beak ; edges plain ; muscular scars very slight. 



Length o'45 inch. Breadth i inch. 



Hah. — North edge of the Great Fisher Bank, about 150 miles N.E. 

 of Aberdeen, in 40 to 50 fathoms. 



I have given the average dimensions, but the largest are just under 

 i^ in. by h in. The central ribs or corrugations vary in degree from 

 being merely faint rays to well-developed corrugations, and they 

 probably serve the purpose of strengthening the shell in that part, like 

 the internal strengthening ribs of Isocardia cor. These riblets show 

 through the shell but are not impressed on the inside, being obviously 

 only surface sculpture. i. 



3. 



Fig. I. — Myrina simpsoni, n. sp. View from the right side. 

 Figs. 2 and 3. — Hinge-line of left and right valves. 



This shell is like, except in size, M. coppingeri, E. A. Smith, -^ a 

 "Challenger " species from Cape York, N. Australia, 1400 fathoms; 

 but this is wider in proportion to length, the lower or ventral margin 



I "Challenger" Pelecypoda, p. 281, pi. xvi, figs. 9, gb. 



