white.] ANNOTATED CATALOGUE. 39 



genera which constitute that family. These facts necessarily point back 

 to a period much more remote than the Laramie, for the origin of the 

 family, and although we lack the. positive evideuce of the fact, we can- 

 not doubt that it not only had its origin as early as the beginning of 

 Mesozoic time, but that it had then become well established. 



The earliest known species belonging to the genus Limncea is L. niti- 

 clula Meek,* which is found in the Bear River Laramie strata of South- 

 western Wyoming. It is represented on Plate G. It is referred to the 

 subgenus Limnophysa Fitzinger. Among its faunal associates is a 

 minute form, which I have described under the name of AceUa halde- 

 mani,] which is represented by enlarged figures on Plate 0. 



Associated with these is also a small Planorbis, belonging to the sub- 

 genus Gyranlus Agassiz. 



. It is a fact which doubtless indicates the imperfection of our present 

 knowledge that, while those two widely differentiated members of the 

 Limnseidse are found in what are regarded as probably the earlier strata 

 of the Laramie Group, the only other species which is referable to the 

 subfamily Lirnnseinse that has been found in any part of that group is 

 a form, which was described by Meek & Hayden, from the Fort Union 

 beds, under the name of Limncea tenuicostata. f Mr. Meek proposed for 

 this form the subgeneric name of Plevrolimncca, but it might probably 

 be referred to Acella with equal propriety. It is represented by a copy 

 of Mr. Meek's figure on Plate 23. Although a greater number of the 

 Planorbinre than of the Limnseinre have been discovered in the Laramie 

 Group, the apparent absence of the latter from most of the fossiliferous 

 strata of that group, even those which contain other palustral pulmonates, 

 such as Physa and Bulimis, is unexpected, and, as before intimated, 

 it is probably not because they did not exist, but because they have not 

 been discovered. 



Mr. Meek described a species under the name of Limncea (Limno 

 physa f) compactilis,§ from Separation Station in Southern Wyoming, 

 which was obtained from strata that he regarded as Tertiary, but which 

 doubtless belongs to the Laramie Group. It probably belongs to the 

 genus Thauinastus, and not to Limncea. It is represented on Plate 26 by 

 figures of Mr. Meek's type specimen. 



Mr. Meek described two species of Limncea, which he stated were dis- 

 covered in the Bridger Group of Eocene fresh-water strata in Southern 

 Wyoming. He named them respectively L. vetusta and L. similis.\\ 

 Both these forms are represented by copies of Mr. Meek's figures on 

 Plate 29. 



From the Green River Eocene Group of Southern Wyoming I have 



*U. S. Geol. Sur. 40th Parallel, vol. iv, p. 181, pi. xvii, fig. 5. 

 tAn. Rep. U. S. Geol. Sur. Terr, for 1878, Part I, p. 84, pi. 30, fig. 9. 

 tSee TJ. S. Geol. Sur. Terr., vol. ix, p. 534, pi. 44, fig. 13. 

 § An. Rep. U. S. Geol. Sur. Terr, for 1872, p. 517. 



|| U. S. Geol. Sur. 40th Parallel, vol. iv, p. 191, pi. xvii, figs. 3 and 4. Also, Simpson's 

 Rep. Great Basin, Utab, p. 367, pi. v, figs. 2 and 3. 



