96 NEW YORK STATE MUSEUM 



Speoific characters. Carapace rather broad and bluntly pod- 

 shaped. Hinge line straight for two thirds the length of the 

 valves, thence sloping downward to the anterior extremity, leav- 

 ing a rostral cleft. 



The anterior end is short and direct; thence the margin turns 

 ventrally, making a blunt angle anterior of the middle of the valve, 

 where this greatest width of the valve occurs. From this angle 

 is a gradual slope upward and backward to the posteroventral 

 angle. The posterior margin is twice the length of the anterior, 

 truncate, usually with a slight outward, but sometimes inward 

 curve. All margins except the dorsal bear a pronounced cord- 

 like rim separated from the general surface by a groove. In 

 young carapaces both rim and groove are quite indistinct on the 

 posterior margin. 



The cephalic region is not distinctly delimited by a furrow, 

 but is marked by a cluster of rather irregular nodes, dorsad of 

 which lies the ocular tubercle, which is small, smooth and more 

 prominent than the rest. The mandibular nodes also are usually 

 crossed by the longitudinal ornament ridges of the surface. 



The ornament consists of series of sharp, ridge-like lines sub- 

 parallel to the length of the shell ; 9 to 12 of these constitute the 

 major series; and they are in a broad sense continuous for the 

 length of the carapace, following a gently undulating course. 

 In the flat intervals between these ridges are much finer ones, 

 which are less continuous and, instead of always being parallel 

 to the major series, may run somewhat obliquely. While the fine 

 lines may at times encroach on the marginal cord, the coarser 

 lines do not, except at the posterior margin. This ornament is 

 subject to some slight variation with growth, and in one speci- 

 men we observe a notable deflection of the riblets at the posterior 

 end of the carapace, which seems to be the result of repaired 

 damage to growth. 



Of these carapaces we have observed 15 to 20 examples, and 

 they vary in dimensions from a length of 11-13 mm and a great- 

 est width of 6 mm to a length of 20 mm and width of 12 mm. 



Rostrum. We have observed a small, leaflike plate among 

 the carapaces, which is marked by a venation unlike anything 



