STREPTORHYNCHUS. 71 



Var. B. Streptorhynchu^ arctostriata. 



PLATE IX. 

 Orthitina arctostriata : Hall: Thirteenth Report on the State Cabinet, 1860. 



Shell semicircular or semielliptical, frequently unsymmetrical, the 

 proportions of length and breadth varying in different individuals : 

 hinge-line straight, nearly or quite equal to or greater than the greatest 

 width of the shell ; sides nearly rectangular to the hinge-line, or 

 curving inwards. Ventral valve more or less convex towards the umbo 

 and sometimes in the middle, curving downwards or flattened towards 

 the front and sides of the shell : beak often distorted ; area vertical 

 or inclined forwards or backwards, usually unequal on the two sides of 

 the foramen, which is closed by a strong convex deltidial plate. Dorsal 

 valve depressed convex, sometimes nearly flat and sometimes very 

 convex, with a narrow linear area : socket-plates strong, and supporting 

 the cardinal process, which is double and has sometimes a faint ridge 

 between the two divisions, which are themselves very short. 

 Surface marked by sharp close radiating crenulated striae, which increase 

 mainly by interstitial additions. 



The shells which I have heretofore referred to this species are represented by 

 individuals varying from one-fourth to three-fourths of an inch in length, and having 

 a width of from one-fourth to one-third greater than the length. 



The examination of a larger collection than I had before me when the species 

 was first described, has ^own that the convexity of the ventral valve is very 

 variable : the extent of the area is likewise subject to much, variation; and the 

 beak is sometimes symmetrical, and varies from extreme distortion to a scarcely 

 perceptible obliquity of the apex. In some specimens there are strong lamellose 

 lines of growth, and in others these lines are scarcely visible. The dorsal valve 

 varies from nearly flat to very convex ; and in some specimens the length and 

 breadth are almost equal, while the cardinal process is scarcely developed beyond 

 the plain socket-plate, which is bent outwards, and sometimes scarcely divided 

 at the apex. The surface striae are, in some specimens, more rounded and pretty 

 equally developed, while in others they are very sharp and unequal. Finally, we 

 can trace the gradation from the unsymmetrical and distorted specimens, to those 

 which are not distinguishable from well-marked specimens of S. pandora as it 

 appears in the Schoharie grit. 



