16 



P. PROLIFICA, n. sp. — Obliquely sub-rhomboidal ; anterior side neariy 

 straight for about half the length, sometimes a little concave near the 

 hinge Kne, and often gently convex, forming an angle of about 75° with 

 the hinge line ; ventral margin uniformly rounded ; posterior side con- 

 cave just below the angle and convex in the lower half. There is, some- 

 times, a small anterior wing, but in many individuals it is either very 

 slightly or not at all developed. Posterior wing moderately compressed. 

 Beak small, scarcely elevated above the area, the latter about one line 

 wide in the left valve and less in the right. The left valve varies from 

 gently to moderately convex. Right valve gently concave. The anterior 

 and posterior sides are sub-parallel. The umbones are between one-fourth 

 and one-third of the length of the hinge line from the anterior angle. 

 Surface with obscure concentric striae and rugose lamellae of growth. 

 Length of a large individual on the hinge line, two inches ; height the 

 same. DiflFers from P. demissa, Conrad, in being in general one-fourth 

 shorter from the hinge-line to the ventral margin. 



Charleton Point and Macasty Bay ; H. R. J. Richardson, T. C. 

 Weston. 



Genus Ischyrinta, N. G. 



Generic characters. — Equivalve, inequilateral, two strong ridges 

 radiating from the beak in the interior of each valve. 



Fig. 4. 



Fig. 4. — Ischyrinia Winehelli. a, left valve ; 6, cast of the interior of 

 left valve ; c, cast of the interior of right valve. 



I. WiNCHELLi, n. sp. — Shell triangular, strongly ventricose. Anterior ? 

 (flat side) somewhat straight, gently convex above and concave below ; 

 posterior ? side slightly curved ; ventral margin moderately rounded. 

 Beaks small and obscure, closely incurved. Surface with a shallow concave 

 groove close to the anterior edge, in which are four or five small rounded 

 ribs all of them covered with very fine longitudinal striae. All the remain- 

 ing portion of the sides of the valves apparently smooth, but in certain 

 lights exhibit indications of minute striae radiating from the umbones to the 

 ventral margin. The anterior or flat extremity is, with the exception of 

 the small wing, gently concave. The anterior and posterior slopes form with 



