244 GEOLOGICAL SURVEY OF THE UNITED STATES TERRITORIES. 



under Corbula, to which it is evidently nearly related. These shells, how- 

 ever, differ from Corbula in having a small tooth on each side of the cartilage- 

 pit, instead of only a single prominent tooth on its anterior side, as well as in 

 having the cartilage-process broad and spatulate. The animal is also said to 

 differ from that of Corbula in having elongated, instead of very short, siphons. 



Until the hinges and interior of a number of fossil bivalves, now only 

 known from external characters, can be studied, it will be difficult to deter- 

 mine the geological range of the genus Corbula with precision. Species 

 have been referred to it from the Carboniferous rocks ; but, so far as I have 

 had an opportunity to ascertain from specimens and the published figures, 

 there seems to be very little probability that they really belong to this genus. 

 The same may also be said even in regard to the few Triassic species yet 

 described under the name Corbula, though it is quite possible that this group 

 may have been represented during that epoch. In the Jurassic, however, we 

 meet with shells apparently presenting the characters of this genus, and its 

 existence in the Cretaceous rocks is well established. The genus was also 

 well represented, through the Tertiary period, during which it nearly or quite 

 attained its greatest development. In the existing seas, however, it still 

 seems but little, if at all, less numerous, and the species are widely distrib- 

 uted. 



The older species appear all to belong to the typical section ; while the 

 type of the proposed subgenus Anisorhynchus occurs in beds belonging to 

 the oldest Eocene or the latest Cretaceous. The species of the group Pachij- 

 odoit came from beds in South America apparently not older than Miocene. 



Corbula crassimarginata, M. & H 



Plate 17, figs. 14, a, b, c. 

 Corbula crassimarginata, Meek anil Hayden (Oct., 1860), Proceed. Acad. Nat. Sci. Philad., VIII, 425. 



Shell very small, short, oval-subtrigonal, rather gibbous ; right valve 

 but slightly larger than the other ; basal border semi-ovate in Outline, the 

 most prominent part being toward the front, rather straighter behind, and 

 very distinctly thickened externally, so as to form a marginal band, which is 

 most prominent on the right valve; posterior side a little longer than the 

 other, and subtruncated at the extremity; umbonal slopes forming a promi- 

 nent oblique ridge from the back part of each beak to the postero-basal 



