1(32 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



seem hardly separable on any well-marked characters from Corbicula, a much 

 greater diversity of form and other peculiarities, than are presented by the 

 living species. 



The type of the group Veloritina is a remarkably gibbous, trigonal shell, 

 with the posterior dorsal margins so strongly incurved, and the umbonal 

 slopes so very prominent on each side, that the posterior dorsal region 

 presents a largely and profoundly sulcate appearance along its whole length. 

 In several respects, particularly in the obliquity of its cardinal teeth and its 

 trigonal solid valves, it agrees more nearly with Velorita than with Corbicula; 

 but it differs materially from the type of the former genus in having its 

 anterior lateral teeth (as well as its posterior) elongate-linear, as in Corbicula. 

 It also differs from both of these groups in the strongly-incurved character 

 of the posterior dorsal margins of its valves, and the depressed nature of its 

 ligament, which is so deeply seated as to be entirely hidden by the very 

 prominent posterior dorsal slopes, in looking at the shell from either side. 



The type of the group Leptesthes, on the other hand, contrasts very 

 strongly in form and general appearance not only with the typical section 

 of Corbicula but also with the section Veloritina; being a transversely- 

 subovate or subelliptic, compressed, extremely thin shell, more like some ot 

 the elongate forms of Crassatella. In form, it agrees much better with some 

 of the transverse existing specie* of Cyrena, such as C. Floridana, Conrad, 

 and C. salrnacida of Morelet, than with living species of Corbicula. When 

 we come to examine its hinge, however, we find that it has the elongated and 

 striated lateral teeth of the latter genus. 



It is not certainly known, I believe, that the genus Corbicula was 

 represented during the Jurassic epoch; though some of the shells that have 

 been figured from rocks id' that age have the external appearance of this 

 genus, and may possibly possess the hinge-characters of the same. Until 

 their hinges can lie seen, however, their relations to this genus must remain 

 doubtful. In the Wealden of Europe, however, there are found numerous 

 species that have been described under the name Cyrena, which, although 

 showing a greater diversity of form than the living species, and apparently 

 some slight differences in the cardinal teeth, seem, in part at least, scarcely 

 distinguishable generically from Corbicula* If those older species, however, 



» Messrs. II. and A. Adams, in speaking of the geographical and geological range of the genus 

 Corbicula, state (in vol. II, p. 447 of their valuable work on the Genera of Recent Mollnsca) that "very 

 numerous extinct forms occur in the Wealden of Europe and America." This, howevor, is an error, so 



