SOME EXTINCT CATTLE 259 
Arctic Circle; but Dr. Hilzheimer^ regards its 
Siberian representative as a separate species, under 
the name of B. primitivus^ its most distinctive 
feature being the greater extension of the parietal 
bones on to the frontal aspect of the skull. Here, 
again, the distinction need not, however, be regarded 
as of more than racial value; and if this view be 
accepted, the Siberian bison should be known as 
Bos prisons priinitivus. 
Before tracing fossil bison into North America, 
brief reference may be made to the Siwalik B. 
sivalensis, which is the earliest known representative 
of the group. In the flatness of the frontal region 
of the skull and the tubular form of the sockets 
of the eyes this species comes nearer to the living 
European than to the American species ; but the 
horn-cores are placed lower down on the skull than 
in either, and there was very probably an extension 
of the parietal bones on to the frontal aspect of the 
skull greater than in the Siberian bison. The 
Siwalik bison may have been the ancestral stock of 
all other bisons, and perhaps also of the Tibetan 
yak, which appears to be an offshoot from the bison 
group. 
As mentioned in an earlier chapter, bison made 
their way into North America by means of a 
former land-bridge across what is now Bering Strait, 
probably during the early part of the Pleistocene 
or later portion of the Pliocene epoch. The fossil 
American bisons have been described in considerable 
detail by Professor F. A. Lucas,^ who recognises no 
1 Op. cit. 1909, p. 254. 
2 "The Fossil Bison of North America," Proc. U.S. Nat, Museum^ 
Washington, vol. xxi. 1899. 
