GEOLOGICAL SURVEY OF THE TERRITORIES. 513 



disposed of. Still, even in that case, it would seem desirable and con- 

 venient to separate them subgenerically, by writing the name Cyrena 

 (Leptesfhes) fracta. If the two groups Cyrena and Corhicula^ however, 

 are to be regarded as distinct genera, there would be nearly or quite 

 as good reasons for so regarding Leptesthes as a genus. 



Locality and position. — Black Butte Station, Wyoming, upper beds. 

 Bitter Creek series. 



CoEBicuLA (Veloritina) Bannisteri, Meek. 



Shell of about medium size, subglobose, the valves being very con- 

 vex and subcircular in outline ; ventral margin forming a regular semi- 

 circular curve; anterior and posterior margins nearly equally, and more 

 narrowly, rounded ; dorsal outline declining distinctly from the beaks, 

 the anterior slope being more abrupt and concave in outline, and 

 the posterior very convex, with the posterior dorsal margins of the valves 

 inflected so as to form a deep, rather broad sulcus wheu the valves are 

 united ; beaks prominent, veutricose, incurved, contiguous, and placed 

 about one-third the length of the shell from the anterior margin ; luuu- 

 lar region impressed ; surface ornamented with fine lines and small but 

 rather distinct, somewhat regularly arranged, concentric ridges and far- 

 rows. (Hinges and interior unknown.) 



Length, 0.87 inch ; height, 0.83 inch ; convexity, 0.G6 inch. 



This is a very neat, symmetrical shell, readily distinguished from the 

 other species yet known from the rocks of this region, by its short, very 

 convex form, and rather distinct, regular, concentric ridges and furrows. 

 In one character it agrees with C. Durl:eei^ from the brackish- water beds at 

 Bear Elver City, more nearly than any other species I have yet seen. 

 That is, in having the posterior dorsal region of its valves strongly 

 inflected, so as to form a deep, broad sulcus or escutcheon along this 

 slope when the valves are united. Its rounded, ventricose form and 

 stronger concentric markings, however, readily separate it from that 

 shell, from which it also differs in being much smaller, while its gibbous 

 form shows it to be an adult shell. It is also related to G. occidentalism 

 M. & H., from the estuary beds near the mouth of Judith Kiver, but it is 

 more rounded in outline, more gibbous, and differs in the inflected char- 

 acter of its posterior dorsal region, its stronger surface-markings, and 

 more impressed lunule. I have not yet seen its hinge or interior, but 

 from its general external characters, I have little doubt that it belongs 

 to the section Veloritina. 



The specific name is given in honor of Dr. Henry M. Bannister, who 

 discovered the only specimen of the species I have seen. 



Locality and position. — Bitter Creek series. Black Butte Station. 

 Wyoming at the same horizon as the large saurian found there. 



CoRBULA UNDiPERA, Meek. 



Shell of moderate size, trigonal- subovate, rather convex, the inequal 

 ity of the valves not being very strongly marked, though always obvi 

 ous; beaks moderately prominent, (that of the right valve being only a 

 little more elevated than the other,) located in advance of the middle, 

 contiguous, and incurved, with a scarcely perceptible forward inclination ; 

 posterior extremity subangular at the connection of its margin with the 

 base; posterior dorsal slope more or less convex in outline; anterior 

 margin rather short and rounded ; base semi-ovate in outline, being 

 most prominent anteriorly, and somewhat straightened behind; left 



33 G s 



