IV. DISTRIBUTION IN TIME. 
A. GENERAL DISCUSSION. 
The American Lymneas are geologically of considerable antiquity, 
the earliest species occurring in the fresh-water strata of the lower 
Cretaceous (Comanchean)! Period, Dr. C. A. White? thus writes of 
the origin and antiquity of the family: “In the Laramie Group, we 
find the family represented by all the principal genera that are known 
among living Limnzide,’ as well as the greater part of the subordinate 
sections of the genera which constitute the family. These facts neces- 
sarily point back to a period much more remote than the Laramie, for 
the origin of the family, and, although we lack the positive evidence 
of the fact, we cannot doubt that it not only had its origin as early 
as the beginning of Mesozoic time, but it had then become well estab- 
lished.” 
Several years after Dr. White penned the above, three Lymnzas 
were discovered in strata then believed to be late Jurassic but now 
referred to the Comanchean Period, thus carrying the family still 
farther back in Mesozoic time. A compilation of the literature re- 
lating to the extinct species of this family at once points to the fact 
that our knowledge of the geological distribution of American fresh- 
water pulmonates is very imperfect. As ponds, swamps and streams 
are believed to have existed in early Mesozoic Time, and also in late 
Paleozoic Time, it is quite probable that the ancestors of the Lymnzas 
lived and flourished at a much earlier period than their discovered 
remains would indicate. There is no apparent reason why Lymnzas 
could not have lived during the Upper Carboniferous (Pennsylvanian) 
Period, as there is already evidence of a well-defined pulmonate fauna.* 
The presence of spiders, scorpions, myriopods and insects, besides 
amphibians, which suggest a fresh or brackish water habitat, all point 
to the existence of conditions not at all unfavorable to the presence 
of Lymnzas and other fresh-water pulmonates, and it is confidently 
believed by the present writer that such evidence will be found at no 
far distant time.° 
Mr. J. B. Woodworth® has made certain pertinent observations 
which will perhaps bear repeating. He says: “We have to note that 
are 29 U. S. Geol. Surv.; Chamberlin and Salisbury, Geology, Vol. III, 
pp. : 9: 
7An. Rep. U. S. Geol. Surv., III, p. 444, 1883. 
*This statement includes Planorbis and some other genera. 
*Pupa vetusta from the Nova Scotia coal measures, Pupa vermilionensis 
and Dawsonella meeki from Illinois. 
5See Chamberlin and Salisbury, op. cit., vol. II, p. 611. 
‘Amer. Geol., XIV, p. 224. 
