McGee and Call — Loss of Des Moines, Iowa. 221 



than the specimen serving as the type.* None of the measure- 

 ments of the Pupadce are given in the table, partly because in 

 attempting to measure them many were broken, and partly. 

 again, because the differences in size appeared to be so small as 

 to be almost inappreciable and hence do not appear to affect 

 the general value of the observations. Succinea seems to have 

 attained a most luxuriant development, and is exceeded in num- 

 bers only by Oligyra occulta. This last species is of remark- 

 able interest from the fact of its recent discovery living in great 

 numbers in the vicinity of Iowa City. In all the recent speci- 

 mens examined, upwards of one hundred and fifty, the 

 of the living forms were larger than the largest of the fossil 

 ones. The two aquatic species, Limnophysa humilis and Lim- 

 ■ sidiosa, present the same depauperate features and lead 

 us to the same general conclusions. Plate V figures both 

 recent and fossil forms of these several species in juxtaposition. 

 the specimens figured illustrating' quite well the most common 

 variations and the relative sizes. (See explanation.) 



In this connection it will be found of great interest to consult 

 the remarks of Semper^ on the influence exerted on animal life 

 by low temperatures. That author experimented on Lymncea 

 . and the results of his long continued observations led 

 to the statement that the Lymnata may be quite frozen up with- 

 out being killed. Extremely low temperature had no influence 

 on the animal life, but entirely prevented growth.^ In this 

 way a permanently diminutive race might arise. At all events 

 the fossil material studied by the junior author, comprising 

 many hundred examples, presents the fact of such depauperate 

 races whatever may have been the cause. 



IV. 



In order to state intelligibly the working hypothesis sug- 

 gested by the foregoing facts, it will be necessary to briefly 

 state the conclusions as to the formation of asar and the deter- 

 mination of river courses reached by the senior author after a 

 practically exhaustive survey of the cenology of the north- 

 eastern quarter of Iowa.§ 



The ice-sheet over this region was thin ; not more than five 

 hundred feet in average thickness. Each pre existing plateau 

 or ridge accordingly produced a relatively considerable attenua- 

 tion of the sheet. ~ Three results followed : (1) The motion of 



, Darwin " On the Origin of Specie 



