130 University of California Publications in Geology [Vol. 9 



la and lb) shows that the horn-eores are almost identical with those of 

 two large males from Raneho la Brea, shown in figures 3a and 3b, and 

 4a and 46, and also with those of the male from the Pilarcitos Valley, 

 California (figured by Rhoads, pi. 12, fig. 2), and the male in Earlham 

 College (figured by Hay, figs. 2 and 3). Viewing figures 2a and 2b, 

 representing a female from Raneho la Brea, it is evident that aside 

 from the smaller size and relatively somewhat more slender form, there 

 is no appreciable difference in the form of the horn-cores. One of 

 the most characteristic and most readily recognizable characters of the 

 species, as has previously been pointed out by Lucas and Hay, is the 

 angle which the basal portion of the horn-cores makes with the longi- 

 tudinal axis of the skull. In Bison bison, and in all other American 

 species so far described, the horn-cores make a more or less obtuse angle 

 with the median line of the facial portion of the skull (see fig 5a), while 

 in B. antiquus they are inserted almost at right angles (figs, la, 2a, 

 3a, and 4a). In B. bison the angle bounded on one side by a line 

 from a point at about one-half the distance from base to tip of a horn 

 core along the middle line of the core to the median line of the skull, 

 and on the other side by the longitudinal axis of the skull, varies from 

 110° to 125°, usually being approximately 120°. In B. antiquus, on 

 the other hand, the angle never exceeds 100°, and is frequently an 

 almost perfect right angle. The widest angle made with the skull in 

 any specimen of B. antiquus so far described is that of a very young 

 animal, little over half grown, from Raneho la Brea, (21188). In 

 this specimen the angle is about 100.° 



Considerable variation exists in the amount of convexity of the 

 forehead, as can be seen by reference to figures lb, 2b and 46. This 

 convexity seems to increase somewhat with age, though there is a great 

 deal of variation which is purely of an individual nature. 



The amount of variation in the size of the lower jaw is remarkably 

 slight, there being only about 5 per cent difference in length from tip 

 to angle in the extremes of five adult specimens, as shown by Table 

 III. The measurements vary to such an extent with age that it is 

 impossible to distinguish males from females, although, judging by 

 analogy with the modern species, there is probably a slight sexual 

 difference in size. 



Comparison of the measurements of the skull and horn-cores shows 

 that B. antiquus differs from both the existing species in the relative 

 breadth of the skull. The length, as shown by several different 

 measurements, is approximately the same for all three species, while 



