8xJ THE CHICAGO ACADEMY OF SCIENCES. 



were living contemporaneously with the last of the Dinosaurs. Yet 

 such is the fact, and the shells of the former are often found com- 

 mingled with the bones of the latter. What were the successive steps 

 in the history of the transmission of these types from that remote time 

 to the present we are unfortunately without the means of knowing 

 with certainty, because of the remarkable paucity of molluscan remains 

 in all the deposits of the great interior region later than the Eocene. 

 All the molluscan remains which have been found in these later de- 

 posits belong to familiar living types, although of extinct species." 



A glance at the accompanying map (figure 8) shows that the 

 American Lymnseas originated, so far as the present records show, 

 in the Rocky Mountain region, and that they continued to live in this 

 region until Pliocene and Pleistocene Time. It is noteworthy that as 

 fast as the new land was exposed and became suitably provided with 

 fresh or brackish water lakes, the Lymnaeas migrated eastward and 

 westward and occupied these areas. The region in which these early 

 Lymnseas are found is believed to have contained vast inland lakes, 

 which, says Dr. White, "for magnitude have now no equal upon the 

 earth." Mr. Clarence King 1 has named these Tertiary lakes as follows : 2 



TERTIARY LAKES. 



EOCENE. 



Middle Province. 



Ute Lake (Vermilion Creek Group, King; Wahsatch Group, Hayden). 

 Gosiute Lake (Green River Group, Hayden; Elko Group, King). 

 Washakie Lake (Bridger Group). 

 Uinta Lake (Uinta Group, Emmons and Marsh). 



MIOCENE. 



Contemporaneous. 

 Province of Nevada and Oregon. Province of the Great Plains. 



Pah-Ute Lake (Truckee Group, Sioux Lake (White River Group, 



King; John Day Group, Marsh). Hayden). 



PLIOCENE. 



Contemporaneous. 

 Province of the Great Middle Province. Province of the Great 



Basin. North Park Lake Plains. 



Shoshone Lake (Hum- (North Park Group, Cheyenne Lake (Nio- 



boldt Group, King). Hague and Hayden). brara Group, Marsh). 



iGeology 40th Parallel., Vol. I, p. 408-458; see Chamberlin & Salisbury's 

 Geology for the present relations of these strata. These Tertiary lakes were 

 recognized at an earlier date by Hayden and Newberry, later by" Powell, Gil- 

 bert, Cope, Marsh and other geologists. See Davis. Proc. Amer. Acad. Arts. 

 & Sci., XXXV, p. 345, for a resume of this subject. Also Paleogeographic man 

 by Willis. Journ. Oeol., XVII, p. 504. 



2 op. cit., p. 458. 



