40 NON-MARINE FOSSIL MOLLUSCA. 



described a species under the name of L. minuscula,* which evidently 

 belongs to the subgenus Leptolimncea Swainson. It is represented on 

 Plate .9. 



The species that have just been mentioned are all that are known to 

 have existed in North America up to the close of the Eocene epoch, which 

 pertain to the comprehensive genus Limnwa. The fresh-water strata, 

 which are known as the White River Group, and are referred to the 

 Miocene Tertiary, are the only strata of that epoch which are at present 

 known to contain any remains of Limnwa. In that group two species 

 of Limnwa have been discovered, namely, L. meeldi Evans & Shumard, 

 and L. slmmardi Meek.t Both of these species belong, apparently, to 

 the typical section of the genus, and both are represented by copies of 

 Mr. Meek's original figures on Plate 32. 



If the non-marine molluscan fauna, of the epoch of which the White 

 River Group is a partial representative, were better known, it may be 

 regarded as certain that we should find in it many representatives of 

 living types; but so far as that fauna is at present known, it is a very 

 meager one, and confined to the pulmonate gasteropods. The apparent 

 entire absence of gill-bearing mollusks in those portions of that forma- 

 tion which have been examined, may, perhaps, have been due to a solu- 

 tion in the waters in which they were deposited, of certain salts or other 

 substances which made them uncongenial for molluscan life. If this were 

 really the case, the presence there of the remains of pulmonate gaster- 

 opods might be accounted for on the supposition that they were drifted 

 into those waters by tributary streams in or upon the banks of which 

 they lived. 



The only other species of Limnwa to be mentioned in this article is 

 the one which was described -by Mr. Meek under the name of L. Mngii, 

 and for which he proposed the subgeneric name of Polyrhitis.% It was 

 obtained from certain strata in Oache Valley, Utah, which are not of 

 earlier date than the Miocene, and which are probably of Pliocene age. 

 Copies of Meek's original figures of this form are given on Plate 

 32. It is an interesting form because of the fact that it is the only 

 known representative of the only extinct subordinate type of Limnwa 

 that is yet known, and because it comes down to a later date than 

 most of the other extinct types of mollusca, of which mention is made 

 in this article. 



Of the other genera of the family Limnaeidse, Planorbis and Carinifex, 

 reuia in to be con sidered. The earliest species of Planorbis yet discovered 

 in JSorth American strata is P. v etur nus Meek & Hayden, § which those 



* Proe. U. S. Nat. Museum, vol. iii, p. 100. 



t For descriptions and figures of these two species see U. S. Geol. Sur. Terr., vol. 

 iv, pp. 598 and 599, pi. 45, iigs. 5 and 6. 



$For description and figures of this species see U. S. Geol. Sur. Terr., 40th parallel, 

 vol. iv, p. 192; and for Meek's diagnosis of the subgenus see U. S. Geol. Sur. Terr., 

 vol. is, p. 532. 



§ Paleontology of the Upper Missouri, p. 107, pi. iv, fig. 1. 



