white] ANNOTATED CATALOGUE. 33 



Length, 39 millimeters; height from base to unibones, 34 millimeters; 

 thickness, both valves together, 2G millimeters. 



The large series of examples of all the described species of Gorbicula. 

 that have been obtained from the Laramie strata east of the Bocky 

 Mountains in Colorado give nnmistakable indications of genetic rela- 

 tionship between them. It has already been stated that G. canliniw- 

 formis is perhaps only a variety of G.fracta; and it may be remarked 

 that G. obesa, G. umbonella, and*', augheyi hold similar relations to each 

 other. It is not unlikely that when all the Corbicukie of the Laramie 

 Group come to be fully studied, some of the species heretofore described 

 must be abandoned ; but in an article like this, it is thought best to make 

 mention of, and to figure, all the forms that have been published by any 

 author, as he has published them. 



The species which Mr. Meek adopted as the type of his subgenus Lep- 

 testhes, namely, G. (L.)fracta* Meek, has been found in considerable abun- 

 dance at Black Buttes station, in Southern Wyoming, and sparingly also 

 at other localities on both sides of the Bocky Mountains. It is figured 

 on Plate 20. 



This is the largest species of the subgenus Leptesthes yet known, and 

 with the exception of C. berthoudi, herein described, the largest species 

 of Gorbicula that has ever been discovered in North American strata. 

 The figures that are here given show it to be a very variable species as 

 regards external form. Fig. 1, on Plate 20, is a copy of Mr. Meek's 

 drawing from his type specimen. The partial truncation of the poste- 

 rior border is a slight deformity of that specimen, and is not common to 

 the species. The dimensions of that figure are also somewhat less than 

 the average for adult shells. Figs. 4 and 5, on Plate 20, represent two 

 views of a young example of ordinary form, but some examples of this 

 species are more elongate transversely. 



]STo examples of the genus Spharium have been found in any North Amer- 

 ican strata of older date than the Laramie Group, and none have been 

 published from that group except the four species which were obtained 

 by Meek & Hayden from the Upper Missouri Biver region, and de- 

 scribed by them respectively under the names Splucrium planum, S. rec- 

 ticardinale, 8. formosum, and S. subellipticum.j These four species are all 

 represented on Plate 17 by copies of Mr. Meek's original figures. The 

 two first-named species are from the Judith Biver beds, and the other 

 two from the Fort Union beds. Some fragments of a species of Sphcerium, 

 which is perhaps different from either of the four species above referred 

 to, have been discovered in the coal-bearing beds at Evanston, Wyom- 

 ing, which beds seem to belong to the upper part of the Laramie Group. 



A few imperfect examples, too imperfect for specific characterization, 

 but evidently belonging to one or more species of the genus Sphcerium, 



* See U. S, Geol. Sur. Terr., vol. ix, p. 161. 

 t.See U. S. Geol. Sur. Terr., vol. ix,pp. 526, 527, pi. 43. 



