,324 PALAEONTOLOGY OF ILLINOIS. 



bous that the convexity of the two valves is greater in the 

 middle than the diameter in any other direction. Hinge line 

 Fi 37 only about equaling half the 



transverse diameter of the 

 valves, and owing to its short- 

 ness, and the convexity of the 

 shell, imparting little or no an- 

 gularity to the lateral slopes at 

 its extremities. Dorsal valve 



Syntrielasma hemipucata. 



Fig. «, dorsal view, natural size, with a part of somewh at more COnVeX than 

 the shell removed to show the diverging socket 



plates; fig. 6, ventral view of same, with a part of iU nthpr flnrl VPT*V strOTlflv 



shell removed to show the three nearly parallel me OWltJl, dJIU. VtUy bUUllgiy 



lamina?, as seen in the internal cast. ■• -• -i n • -i 



arched ; umbonal region gib- 

 bous, and often in adult shells projecting beyond the beak of the 

 opposite valve ; beak strongly incurved, so as to bring its apex 

 under the beak and nearly against the area of the other valve ; 

 area about half as high as in the other valve, and distinctly in- 

 curved with the beak. Ventral valve convex; beak only moder- 

 ately prominent, and arched or more or less incurved; area small, 

 about one-third as high as wide, not always very distinctly defin- 

 ed, arching with the beak; its triangular foramen scarcely as wide 

 as high. Surface of both valves ornamented with rather fine, 

 regular, crowded radiating striae, and a few very large radiating 

 plications, never defined on the umbones, but becoming more 

 prominent towards the front, where they terminate in deeply 

 interlocking, angular marginal projections. Of these plications 

 there are two — rarely three — on each side of the slightly larger 

 and more prominent one forming the mesial fold of the dorsal 

 valve ; while on the ventral valve there are three — rarely four 

 — on each side of the mesial sinus. A few zigzag marks of 

 growth also traverse the anterior and lateral margins of the 

 valves, parallel to their deep'ly notched edges. Length of a 

 medium-sized adult specimen, 0.72 inch; breadth of ditto, 0.74 

 inch; convexity, 0.75 inch. Radiating striae numbering twelve 

 to fifteen in 0.10 inch. 



