LYMN^EIDJE OF NORTH AMERICA. 81 



the fresh-water Mollusca of North America attained approximately 

 their present characters during the period of evolution of the base- 

 leveled lowlands, and that they have undergone little modification in 

 the succeeding periods until now. 'To so great a degree had this 

 differentiation then attained/ states Dr. C. A. White in his report on 

 the Cretaceous invertebrates of the Plateau Provinces, 'that the species 

 of Unio,; Helix, Physa, etc., seem to have been as diversified and well 

 developed as they are at the present time. Indeed, the species of these 

 genera are so closely like some of those now living that they need only 

 the fresh condition of recent shells to remove all suspicion of their 

 great antiquity from the mind of the casual observer.' " 



It is interesting to note that in the earlier Cretaceous beds the 

 mollusks are, with one exception, either naiades or aquatic pulmonates, 

 while in the Laramie beds of the Upper Cretaceous the fauna has 

 increased to such an extent as to include a large percentage of the 

 existing families of American fresh-water mollusks as well as several 

 families of land shells. The difficulty of assigning these fossil species 

 to the proper genus is well illustrated by the history of the following 

 species : 



In 1878 Dr. White published the description of a mollusk under 

 the name of Acella haldemani, which was supposed to be congeneric 

 with Acella gracilis Hald., a recent mollusk, which the fossil shell 

 markedly resembled in form. 3 Later, 4 by the examination of additional 

 material and by improved methods of preservation, it was found that 

 the columella of this species bore two distinct plaits. This discovery 

 necessitated the removal of the species from the Lymngeas, no species 

 of which has ever been seen with two columella plaits, and, as there 

 was no available group in which to place this peculiar shell, a new 

 genus was erected by Dr. White for its reception, which he called 

 Tortacella, and which he placed in the family Auriculidse. Lymnaa 

 micronema, of the same author, is doubtful and will perhaps be found 

 to belong to the same genus as haldemani. There are one or two other 

 species of doubtful affinity, owing to their imperfect state of preser- 

 vation. 



Dr. White says 1 in "reviewing the collections which represent the 

 fossil faunse herein discussed, so many familiar forms are seen that 

 ic is difficult to realize the fact that a large proportion of them, including 

 those especially which have been mentioned by name in this article, 



3 Ann. Rep. Geol. & Geog. Surv. Terr., XII, pt. I, p. 84, pi. 30, fig. 9, 1878. 

 *Bull. U. S Geol. Surv., 128. p. 44, pi. 5, figs. 8-12, 1895. 

 'Amer. Journ. Sci., ser. iii, Vol. XX, p. 48, 18S0. 



