512 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



CORBICULA'? FRACTA, VCtr. CEASSIUSCULA, Meek. 



CorMcula ? fracta, Meek, 1870. Hayden's Annual Report for that year, page 314. 



This shell agrees so very closely in form aud size, as well as in its 

 hinge and pallial and muscular impressions, surface, characters, &r.., 

 ■with the species I have described in the report above cited, from the 

 shale over one of the Hallville beds, under the name G orb icula 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 5 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 0.113 inch in 

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

 specimens being generally more or less flattened between the laminag 

 of the shale, it is difiicult 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 the same species from more calcareous deposits ; but I 

 have never seen a difference of this kind so stro]igly marked in specimens 

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

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

 name crassiuscula, w^hich it can retain if further comparisons should show 

 it to be specifically distinct. 



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

 ence between it and the characteristic forms of CorMcula and Gyrena, 

 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 forna. 

 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 important differences, of which I had seen 

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

 specimens then seen were not sufficiently well preserved to ]3ermit 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 Gorhicula and Gyrena, the anterior lateral tooth differs from 

 that of Gyrena in being linear and elongated parallel to the hiuge-mar- 

 gin, as well as slightly striated, thus agreeing with the corresponding 

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

 hand, is more nearly as in Gyrena, being shorter than in Gorhicula, 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 Gorhicula, though less distinctlj'. 

 Again, the ligament is also decidedly longer than in Gorhicula, or than is 

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

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

 subsemicircular sinus. 



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

 of both Gyrena and Gorhicula, without agreeing exactly with either. It 

 is well known to x)aleontologists, 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 Gorhicula, but place the whole under Gyrena. 

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



