24'.) 



Dorsal valve more syniiuotrical than tlie ventral, the convexity being either 

 moderate, or strong and reguhir, though sometimes the valve ia more or les3 

 depressed-convex, or flattened in the middle, often with a tendency to forma 

 shallow, undefined depression or sinus, commencing a short distance in front 

 of the beak, and extending towards the front margin, broadening gradually at 

 the same time. This sinus ia found only in the more depressed specimens, 

 and, when deepest, it forms only a slight undulation of the surface. The valve 

 is most prominent at or posterior to the middle ; in the former case, the curve 

 from the depressed beak to the front is very regular, but in tlie latter case, it 

 arches up somewhat rapidly from tlie beak, and then slopes off more gradually 

 towards the anterior margin. Across the middle of the valve, from side to 

 side, the curve is seldom regular, the surface generally rises with but little 

 curvature for a varying distance from the margin, and then extends straight 

 across the center, or, if the specimen have a sinus, it is there slightly de- 

 pressed. The cardinal extremities are more or less flattened, and this flattened 

 area sometimes extends along the cardinal margins, narrowing to a point near 

 the beak. A line from the cardinal extremities to the middle of the valve, 

 forms a slight sigmoid curve. Beak depressed and not extending behind the 

 hinge line, which is straight, and, in a single instance, slightly extended be- 

 yond the sides of the valve. Cardinal process small, thin, bitid above, with the 

 two small processes on each side projecting backwards. A small projection in 

 the center below, extends a little forward and towards the ventral valve. 

 Socket plates short, thin, very divergent, forming an angle of about 135 

 degrees. 



The surface of both valves is marked by very fine, rounded, thread-like or 

 sharp, raised lines, increasing in number by intercalation, and probably in 

 some cases by bifurcation also. The interspaces are slightly flattened and 

 broader than the lines. One set of raised lines commences at the beak and 

 extends to the margin, each line being exceedingly minute at the beak, but 

 increasing very gradually- in size. The intercalating lines generally begin to 

 come in posterior to the middle, and thence, to the front and sides, new ones 

 are continually being added, until the number at the margin is about double 

 that near the beak, there being generally but one intercalating line for every 

 interspace commencing at the beak. In some specimens concentric lines of 

 growth are faintly preserved. 



There seems to be no limit to the degree of variiitiou which a 

 species of Sireptorhynchus may assume. It may be symmetrical in 

 some specimens and u asymmetrical in others. The margins may 

 differ much in outline, and the hinge area vary in width from a 

 few lines to several inches. The beak may or may not be extended, 

 and (uriK'd and twisted to an enormous extent, and the surface 

 markings may be fine or coarse. Tints Ave have no limited and 



Bl'L. BfF, SOC. N.\T. SCI. (32) .TANUART, 1874. 



