44 



upper end ; posterior muscular impression broader and more shallow ;- 

 pallia! line usually well defined, and provided with a shallow, rounded, or 

 semicircular sinus ; hinge rather strong, with the three cardinal teeth 

 well developed in each valve, the anterior two of the left valve, and the 

 posterior two of the right, being more or less sulcated ; anterior lateral 

 teeth long, linear, and not very prominent ; posterior shorter and very 

 remote from the cardinals ; both anterior and posterior laterals very 

 nearly smooth, or minutely granulo-striate. 



Length of a medium-sized adult specimen, 1.62 inches ; height, 1.28 

 inches ; convexity, 0.92 inch. Some fragments indicate about one-third 

 greater size for tile largest. 



Along with the specimens in which the foregoing characters were 

 observed, there are a few others of smaller size, that are proportionally 

 shorter and more nearly trigonal in outline, the beaks being also more ele- 

 vated, central, and not at all flattened or eroded. These probably belong 

 to a distinct species; but as the typical specimens of the species just 

 described vary considerably in form, and all are in a very unsatisfactory 

 condition of preservation, I have some doubts in regard to the propriety 

 of separating the shorter form as a distinct species. Should additional- 

 collections, in a better state of preservation, show this form to be dis- 

 tinct, however, it might be called G. [LeiHestlies) utnbonella. 1 suspect 

 that there are still one or two other species represented among the nu- 

 merous broken and distorted specimens in the collection, but the material 

 is not siifficient to settle this question. 



The species here described seems to be most nearly allied to some of 

 the shorter of the forms described by me in Dr. Hayden's Sixth An- 

 nual Eeport, page 512, doubtfully as a variety of C. fracta, under the 

 name C ? fracta, var. crass hi scula, but which are j^robably distinct from 

 C. fracta. At any rate, the species here described is a still shorter, 

 more nearly equilateral, and generally more gibbous shell, which also 

 usually differs in the peculiar flattening of its umbones. All of these 

 shells, however, as well as the typical C. fracta, agree closely in their 

 hinge-characters, and form a very peculiar group. 



In first proposing the subgeneric name Leptesthes for the reception of 

 C. fracta and the sui)posed variety crassluscula, from the Bitter Creek 

 beds at Hallville and Black Butte. Wyo., I called attention to the fact 

 that these shells seem to combine the characters of both Cyrena and 

 CorhicuJa. In the elongation and striation of their lateral teeth, partic- 

 ularly the anterior laterals, they agree most nearly with Corbicula ; but 

 in the much more transverse form of the shell in most cases, as well as 

 in size and general appearance, they agree better with the Cyrenas, par- 

 ticularly with such species as Cyrena Florldana, Conrad ; C. colorata, 

 Prime ; and G. salmacida, Morelet, than with at least the existing forms of 

 Gorbicula. The name Leptesthes was suggested by the extreme thinness of 

 the typical species G. fracta ; but we now know that this character is 

 not constant in the group, the other species being generally quite thick. 

 Nor is the very transvere form of the typical species, aud the species or 

 variety G. crassiuscitla, a constant character, some of the others now be- 

 fore me from Colorado being as short as several existing species of Gorbi- 

 cula. These shells, however, present certain peculiar hinge-characters 

 that seem to be constant. These are the narrow, elongated form of the 

 anterolateral teeth, like those of Gorbicula, and, at the same time, th& 

 rather narrow but shorter form, and remote position of the posterior lat- 

 erals, more like those of Gyrena, excepting that they are separated from 

 the cardinal teeth by a more or less broad-flat space in each valve, fitting 

 closely together when the valves are united. In the striation of th& 



