THE NEBRASKAN ICE INVASION 245 



Reviewing the biota, it is observed that 14 species of plants are represented 

 and 75 species of animals, distributed as follows: 



Mollusks 50 species 



Insects (wings) x " 



Vertebrates 25 " 



Of these all are now living with the exception of a large part of the mammals 

 of which 23> or 92 percent are extinct. The mammalian fauna resembles most 

 closely the fauna of the Equus zone or Sheridan formation described by Osborn. 

 It likewise resembles the fauna of the bone caves of Pennsylvania as well as 

 the Hay Springs fauna of Nebraska and Hay has stated his belief that all of 

 these deposits should be referred to the Aftonian. 95 There is clearly a very 

 close resemblance between the species represented, and particularly between 

 the extinct species, but this would most likely be the case in the first inter- 

 glacial interval, not only for the reason that the large mammals could find 

 refuge south of the ice sheet, but also because all of the species undoubtedly 

 lived in abundance in the region south of the affected territory, where the en- 

 vironment was possibly but li€tle changed, and formed a reserve fauna which 

 migrated northward as soon as the Aftonian climate became favorable. 



35 Smith. Mis. Coll., LIX, p. 15. 



