Wetter — Rinderhook Fanned Studies. 97 



of the middle; posterior margin nearly vertically truncate 

 below, meeting the ventral margin in a rounded right angle, 

 and obliquely truncated above, this upper portion meeting 

 both the hinge-line above and the lower portion of the 

 posterior margin below, in obtuse angles, giving to the pos- 

 terior margin of the shell a very angular outline. Beak 

 elevated above the hinge-line, flattened, incurved, directed 

 forward, situated at a point about one-fifth the length of the 

 shell from the anterior extremity. From the beak to the 

 postero-ventral angle there is a prominent, angular umbonal 

 ridge, with another similar but less conspicuous ridge extending 

 from the beak to the angle in the middle of the posterior 

 margin. An ill-defined, shallow but rather broad sinus, which 

 is sometimes a mere flattening of the shell, extends from the 

 beak obliquely to a point in front of the middle of the ventral 

 margin. The greatest convexity is on the umbonal ridge at 

 about the middle of the shell. Surface marked by rather 

 sharp, irregular, crowded, concentric lines of growth. 



IlemarTcs. The specimens indicated as the " type," of this 

 species in the University of Michigan collection are two in 

 number, a larger one imperf ectly preserving both valves, and 

 a smaller, nearly perfect left valve. Hall's illustration* of the 

 species was drawn from the larger one of these two specimens, 

 but the figure is to a large extent restored with no indication 

 of the restored portions. The figure here published is based 

 upon the second of the two types above mentioned. The 

 larger specimen is 38 mm. in length and has sharper and more 

 irregular lines of growth. 



Sphenotus bicarinatus (Win.). 



PI. IV. f. 10. 



Edmondia bicarinata. Bull. U. S. G. S. No. 153: 242. 



Shell small, strongly convex, subelliptical in outline; length 

 13 mm., breadth, 6^ mm. Hinge-line straight posteriorly, 

 bending downward in front; anterior margin short, rather 

 sharply rounded; ventral margin slightly arcuate, subparallel 

 with the dorsal margin ; posterior margin rounded. Beaks a 

 little elevated above the hinge-line, flattened, incurved, directed 



* Pal. N.Y. h\pl. 66. f. 14. 



