STREPTORHYNCHUS. 
71 
Var. B. Streptorhynchus arctostriata. 
PLATE IX. 
Orthisina 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 shown 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 strise 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. • 
