LYMNZID# 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.* Later,* 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 Lymneeas, 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 
Yortacella, and which he placed in the family Auriculide. Lymnea 
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’ in “reviewing the collections which represent the 
fossil faunze 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, 
’Ann. Rep. Geol. & Geog, Surv. Terr., XII, pt. I, p. 84, pl. 30, fig. 9, 1878. 
‘Bull. U. S Geol. Surv., 128, p. 44, pl. 5, figs. 8-12, 1895. 
1Amer. Journ. Sci., ser. iti, VV Ol, koxp ae IR sO: 
