496 University of Kansas Geological Survey. 



extremity small, truncated, or narrowly rounded ; basal margins 

 nearly straight, connected by a ventral accessory plate similar 

 to the dorsal one, except that it is shorter, broadest behind, but 

 coming to a slender point in front about midlength the shell, 

 longitudinally divided by a linear groove ; front regularly 

 rounded both vertically and laterally ; anterior gape consisting 

 of a narrow vertical slit, which occupies the middle of a some- 

 what prominent projection at the antero-basal portion of the 

 shell, which projection has the shape of a Norman shield, as 

 seen by front view when both valves are in their natural posi- 

 tion, and which seems to have been occupied by a much wider 

 gape in the younger than in the adult condition of the shell ; 

 both umbonal grooves distinct, both upon the outer surface and 

 upon that of the stony cast ; anterior grooves broader and deeper 

 than the other, but both are slender ; besides the two umbonal 

 grooves there is another somewhat broader groove or furrow, 

 extending with a broad downward curve from the posterior side 

 of the beak to the posterior end of the shell. This groove, like 

 the others, is distinctly traceable upon the outer surface, but is 

 more distinctly seen upon the stony cast. A broad, subcircular, 

 cake-like umbonal accessory valve covers the beaks and the 

 space between them, the valve being divided by a suture into 

 two nearly semicircular pieces so neatly that it is hardly per- 

 ceptible until the valves are slightly displaced. The margins of 

 the principal valves between the beaks and the Norman-shield- 

 shaped projection are narrowly but abruptly averted, which, 

 with the beaks above and the borders of the projection below, 

 bound a distinctly hollow space each side and below each beak. 

 Besides the grooves before mentioned, the surface is marked by 

 fine concentric, distinctly raised lines on each side of the shell, 

 but they are less distinct upon the surface of the Norman-shield- 

 shaped projection than elsewhere. Between the posterior 

 grooves or furrow before mentioned as ending at the posterior 

 margin of the shell and the dorsal margin, the surface is occu- 

 pied by strong, irregular scales and laminse that were succes- 

 sively increased as the shell increased in size. Length, thirteen 

 mm. ; greatest height, seven mm. ; breadth at front, six mm." 



