486 THE AGE OF MAMMALS 



as Felis atrox, while a larger Pleistocene form in California was named F. 

 imperialis. Confirmation of its character has recently been obtained ^ in 

 the Asphalt Beds of Rancho La Brea. The skull {Felis atrox behbi) is 

 remarkably similar to that of the existing African lion and the cave lion 

 of the European Pleistocene. This California variety approaches closely 

 in measurements Leidy's type jaw from Natchez, Mississippi, with some 

 specific variations. The muzzle is very wide compared with the length of 

 the skull. As in the European form, the superior outlines of the skull 

 approach the lion more closely than the tiger. The related form, F. impe- 

 rialis, from Livermore Valley, California, associated with remains of 

 B. latifrons, and of Elephas, Equus, and Canis indianensis, possibly occurs 

 in the Potter Creek Cave - and is reported at Washtucna Lake, Washington. 

 It is undetermined whether it is really a distinct species. 



Summary. — The mammals which have been described above as con- 

 temporaneous wAih Megalonyx in such widely scattered regions as Pennsyl- 

 vania and California appear to constitute a great group adapted to tem- 

 perate and north temperate conditions of climate. Except in the probably 

 successive deposition of Big Bone Lick, not a single boreal, tundra, or 

 steppe species occurs among them; on the other hand, there is a large 

 element of hardy species of southern affinity, such as the sloths. 



3. The Fauna of the Ovibos Zone, Perhaps Corresponding with 

 THE Arctic and Tundra Period in Europe 



This is an impoverished fauna, reduced in numbers and in variety. Full 

 of modern or existing species, far to the south of their present range. 



We thus enter a new faunal zone, Avhich may be called the zone of Ovibos, 

 or the musk ox. The advent of this northern form in the central United 

 States, as shown in the accompanying figure, is perhaps coincident with the 

 period of the last great glacial advance which is recorded in the great terminal 

 moraine. The glaciated, or partly glaciated areas of the United States at 

 this period of maximum advance are here mapped as recorded by Dana. 

 The kno^vn southerly distribution of the musk ox in Pleistocene times is 

 independently plotted from various records, yet it appears to coincide in 

 the most remarkable way with the southerly boundaries of the great ice 

 sheet. (See Fig. 214.) The mammals of this period of maximum glaciation 

 and of the subsequent recession of the ice may be considered together as 

 constituting the third great life zone of the American Pleistocene, which 

 may correspond with the third life zone in Europe. 



It by no means rests upon the same positive or cumulative evidence as 



1 Merriam, J. C, The Skull and Dentition of an Extinct Cat Closely Allied to Felis atrox 

 Leidy. Univ. Cal. Puhl., Bull. Dept. GeoL, Vol. V, no. 20, Aug., 1909, pp. 291-304. 



2 Bovard, J. F., Notes on Quaternary Felidse from California. Univ. Cat., Puhl., Bull. 

 Dept. GeoL, Vol. V, no. 20, Sept., 1907, pp. 155-166. 



