512 GEOLOGICAL SURVEY OF THE TERRITORIES. 



CoRBicuLA? FRACTA, imr. CRASsiuscuLA, Meek. 



Corhicula? fracta, Meek, 1870. Haydeii's Annual Report for that yeai-, page 314. 



This shell agrees so very closely iu form and size, as "well as in its 

 hinjiv and pallial and muscular impressions, surface, characters, &c., 

 ■svith the species I have described in the repoit above cited, from the 

 shale over one of the Hallville beds, under the name Corbicnia fracta, 

 that it hardly seems proper to separate it specifically. Yet, in the thick- 

 ness of the substance of these shells from the two localities and horizons, 

 there is a very marked difference; those from Hallville being extremely 

 thin, even in the largest specimens, the thickness not measuring more 

 than from 0.02 to 0.03 inch, while in examples of corresponding size of 

 those here under consideration, it measures from 0.10 to 0.12 inch in 

 thickness. The latter also seem to be more convex, but the nallville 

 specimens being generally more or less flattened between the laminte 

 of the shale, it is difficult to know exactly how far this want of convexity 

 may be due to accidental pressure. 



I am aware that shells found in argillaceous shales are usually thinner 

 than examples of tlie same species from more calcareous deposits ; but I 

 Lave never seen a difference of this kind so strongly marked in specimens 

 certainly known to belong to the same species. This thicker shell is, 

 therefore, placed here provisionally as a variety of C.fractn^ under the 

 name crassiuscula, which it can retain if further comparisons should show 

 it to be specitically distinct. 



In describing the species C. fracta, I noticed several points of differ- 

 ence between it and the characteristic forms of CorhicuJa and C'yreua, 

 and suggested for the group of which it may be regarded as the type 

 the subgeneric name Leptesthes. The peculiarities mentioned were the 

 extreme thinness of the shell, and its very elongated depressed form. 

 The specimens here under consideration show that the thinness of the 

 shell is not a constant character, though they at the same time show that 

 this type presents other more im]io)'tant differences, of which I had seen 

 indications before, but which I did not mention especially, because the 

 specimens then seen were not sufficiently well ])reserved to permit these 

 characters to be clearly defined. They are differences in the hinge. 

 For instance, although the primary teeth do not differ materially from 

 those of Corbicida and Gyrena, the anterior lateral tooth differs from 

 that of Cyrcna in being linear and elongated parallel to the hinge-mar- 

 gin, as well as slightly striated, thus agreeing with the corresi)onding 

 tooth of Corhicnla. Its posterior lateral tooth, however, on the other 

 hand, is more nearly as in Cyrena, being shorter than in Corhicnla, and 

 placed very remote from the cardinal teeth, while the intervening cardi- 

 nal margins are wide, flat, and, when the valves are united, close fitting. 

 Yet this tooth is also striated as in Corhicula, though less distinctly. 

 Again, the ligament is also decidedly longer than in Corhicnla, ox Wi-^n is 

 usual in Cyrena, and also less prominent, there being apparently no 

 elevated iulcrum for its attachment. The pallial line shows a shallow 

 subsemicii'cular sinus. 



It will thus be seen that these shells combine some of the characters 

 of both Cyrena and Corhicnla, without agreeing exactly with either. It 

 is well known to paleontologists, however, who have studied fossil- 

 shells of these groups, that there are many species that show interme- 

 diate characters between these genera, so that some eminent authorities 

 do not admit the genus Corhicnla, but place the whole under Cyrena. 

 Should this view prevail, the forms here under consideration might be so 



