EXTINCT BISONS OP NORTH AMERICA— HAY. 



187 



A comparison of the teeth of B. alleni, as represented in the skull 

 here described, shows that they do not differ in structure essentially 

 from those of Bison hison. They are, however, considerably larger 

 than those of most specimens of the latter, but the 

 hinder lower molar is hardly larger than that of No. 

 172689, U.S.N.M., from northern Alberta. 



Figure 8 represents the true molars of the left side of 

 the upper jaw. In m.^ the internal column between the 

 two lobes had been worn dowai to where it joined the 

 enamel suiToundmg the crown; in m.^ this point had not 

 yet been reached; m m.^ the summit of the column had 

 just begun to wear. The enamel surrounding the cement 

 lakes has a very sunple structure and this will enable us 

 to distinguish these teeth usually fi'om those of B. regius, 

 the next species to be described. So far as the ^v^iter can 

 now judge, the teeth of B. alleni may usually be distin- 

 guished from those of B. hison by then- greater size. 

 Figure 9 gives a view of the lower teeth of the left side. 



It may be profitable to determine certam mdices for 

 the purpose of showmg the proportions of the skulls of 

 bisons, as Osboni has proposed ^ for the horses. To obtain 

 the cephalic mdex, the width at the rear of the orbits, 

 multiplied by 100, is divided by the basilar length. The 

 facio-cephalic index is obtained by dividing the dis- 

 tance from the front of the premaxillse to the middle of 

 a Ime jommg the rear of the orbits, multiplied by 100, 

 by the basilar length. The cranio-cephalic hidex is 

 determmed by dividing the cranial length- (distance 

 from the middle of the occipital crest to the middle of 

 the straight Ime joining the rear of the orbits, multiplied 

 by 100) by the basilar length. In the case of the skull 

 of No. 7706, here described the folio wmg results are 

 secured : Cephalic mdex, 59.4 ; facio-cephalic index, 72,1; 

 cranio-cephalic mdex, 41,4. 



The atlas. — This bone (pi. 17, fig. 2) is wholly unin- 

 jured, except that a little of it has been broken off at fig. 9.— bison al- 

 the right hmder angle and a very little from the hmder ''^'^- left low- 



, T p 1 , p T . ER PREMOLARS 



border or the left hmder angle. Among the atlases and molars of 

 described by Richardson, by Tscherski, by Dr. J, A, no. 7706 united 



T Tvr * o ' -^ T J States Nation- 



Allen, and JVlr. A. Stewart are some which have prac- al mcseum. 



tically the same size as that of the Alaskan bison ^^' 



here described; but the proportions are in some cases different. 



1 Mem. Amer. Mus. Nat. Hist., n. s., vol. 1, p. 57. 



2 In order to determine easily the cranial length it is necessary only to And mechanically the perpen- 

 dicular of a right-angled triangle whose base is one-half of the distance between the rear of the orbits and 

 whose hypothenuse is the distance from the occipital protuberance to the rear of an orbit. Similarly may 

 be determined the facial length. 



