366 THE AGE OF MAMMALS 



The general shape of the carapace is rather hke that of the Sclerocalyptus 

 than of Glyptodon. As in the Pampean Panocthus, the seventh to the 

 fifteenth rows of lateral plates of the carapace are movable, or imbricating. 

 Taken altogether this animal combines characters of several of the South 

 American forms of the Miocene and Pleistocene periods. 



The most important member of the Proboscidca is the Mastodon (Stego- 

 don) mirificus, represented by a complete skull with the upper tusks pre- 

 served. This animal has been found in Nebraska, Idaho, and Texas. 

 The upper tusks are round, without enamel, upturned, and divergent; 

 the lower jaw is short and tuskless. In old age only four grinding teeth, 

 the last upper and lower teeth on each side, are in use at one time. These 

 third molars have five to six low cross crests, and are thus in a stegodon 

 stage of evolution, but the crests are partly interrupted by supplementary 

 median tubercles, so that it does not appear that this annual is closely 

 related to any known species of the Asiatic Stegodon. A number of other 

 species of mastodons were named by Cope from this formation from single 

 molar teeth. It is thus uncertain whether they belonged to trilophodont 

 or tetralophodont, long- or short-jawed series. The provisional references 

 are T. shepar-di, T. tropicus, T. precursor, T. humboldti. 



Among the Carnivora a problematic feature is the earliest record of 

 the genus Felis in the species F. hillianus, which may, however, be wrongly 

 referred. The only musteline known is Canimaries cumminsii, an animal 

 very remotely related to the marten, weasel, etc. The giant Carnivora 

 of the period include the amphicyon-like Borophagus, also possibly a true 

 species of Amphicyon. 



Upper Pliocene or Lower Pleistocene 



Equus and Elephas Zone 



Considerable uncertainty again exists whether the formations here 

 recorded should be arranged under the Upper Pliocene or under the Lower 

 or initial stage of the Pleistocene. 



The evidence afforded in Florida by the Peace Creek Formation is 

 again singularly welcome, because we here find an Equus and Elephas 

 fauna which is said to underlie marine beds of Upper Pliocene age. These 

 geologic facts are so important in the chronology of American mammalian 

 life that they may be described first. 



Peace Creek Formation, Florida 



There extends for miles along Peace Creek in Manatee County, south- 

 western Florida, a series of beds which Dall ^ has termed the Peace Creek 



' Dall, W. H., and Harris, G. D., The Neocene of North America. Bull. U.S. Geol. Surv., 

 no. 84, 1892. 



