IV. DISTRIBUTION IN TIME. 

 A. GENERAL DISCUSSION. 



The American Lymnseas are geologically of considerable antiquity, 

 the earliest species occurring in the fresh-water strata of the lower 

 Cretaceous (Comanchean) 1 Period. Dr. C. A. White 2 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 Limnaeidse, 3 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 Lymnaeas 

 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 Lymngeas 

 lived and flourished at a much earlier period than their discovered 

 remains would indicate. There is no apparent reason why Lymngeas 

 could not have lived during the Upper Carboniferous (Pennsylvanian) 

 Period, as there is already evidence of a well-defined pulmonate fauna. 4 

 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 Lymnaeas 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. 5 



Mr. J. B. Woodworth 6 has made certain pertinent observations 

 which will perhaps bear repeating. He says : "We have to note that 



1 BuU. 29 U. S. Geol. Surv.; Chamberlin and Salisbury, Geolog-y, Vol. Ill, 

 pp. 66, 119. 



2 An. Rep. U. S. Geol. Surv.. Ill, p. 444, 1883. 



•This statement includes Planorbis and some other genera. 



4 Pupa vetusta from the Nova Scotia coal measures, Pupa vermilionensia 

 and Dawsonella meeki from Illinois. 



B See Chamberlin and Salisbury, op. cit., vol. II p. 611. 



"Amer. Geol., XIV, p. 224. 



