
998 The American Naturalist. [ November, 
vex anteroposteriorly, as in the U. sfe/eus, but it is regularly convex 
transversely, which it is not in that species. The two species of Ursus 
named have relatively larger molar teeth than in the other species o 
f 
the genus, but in the Californian cave bear they are relatively still 
larger, and especially broader, having a swollen area between the 
tubercles not recognizable in the m. 1. in those species, and but feebly 
in m. 1. The canines are also relatively larger, judging from the size 
of their alveoli. Another peculiarity is the presence of three infra- 
orbital foramina. Ameghino,! represents two in the 4. dona@rense. 
In dimensions this skull equals that of the largest grizzly bears, 
and the average of the European cave bears. Some of the latter 
exceed it in length, but the form in the 4. simum is more robust 
than in either of those species. To judge by the skull alone, the 
Californian cave bear was the most powerful carnivorous mammal which 
has lived on our continent. Its short nose and full rounded forehead 
must have given it a peculiar physiognomy. The living mammal 
which approaches nearest in general appearance is probably the rare 
` black-and-white bear of Thibet, the Æuropoda melanoleuca of Milne- 
Edwards, which connects the Arctotheria with the extinct Hyzenarctos 
of the Neocene ages. It was a much larger animal than the 4. me/ano- 
leuca. Unfortunately we can form no idea as to the color of its fur. 
Like its South American congener, the Californian Arctotherium 
was associated with gigantic sloths (Mylodon), and it belongs to a 
fauna which has left in other localities in North America Megatheriums 
and Glyptodonts. In other words, it is one of the forms which justify 
-the statement which I have made elsewhere (Proc. Phila. Acad., 1867, 
p. 156; Proc. Amer. Philos. Soc., 1871, p. 99), that during the late 
Pliocene or early Plistocene an invasion of Mammalia from the south 
took place. I have suspected that this invasion originated after the 
north had been covered by an ice sheet which prevented immigration 
from Asia and permitted it from the south, since no predecessors of 
the southern types of Mammalia had been found at that time in older 
North American horizons. Since that was written no ancestral forms 
of the Megatheriidz and Arctotherium have been found, but ancestors 
of several other members of the South American fauna have been dis- 
covered, Thus a genus of Glyptodontide (Caryoderma Cope) has 
been found in the Upper Miocene (Loup Fork) of Kansas; and a 
primitive type of peccaries (Bothrolabis Cope) has been obtained from 
the middle Miocene (John Day) of Oregon. Primitive forms of tapirs — 
occur in the Upper Miocene (Loup HOR genus ———— eie 
l Mamm, Foss. Argentinos, Pi. III, ne: I. 

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