1916] Chandler: Skull and Dentition of Bison antiquus 



123 



of variation in length of horn-cores, considering only those specimens 

 which are estimated to be ten years of age or older, numbering twelve, 

 is about the same as that of the skull, 20 per cent. The basal circum- 

 ference of the horn cores is from 10 per cent to 45 per cent greater 

 than the length along the upper curvature. The size of horn-cores 

 seems to vary much more in young animals, sometimes attaining the 

 full growth at an age of four to six years, at other times being far 

 below average size at this age. The variation in amount of curvature 

 of the horns is unexpectedly great, but the general direction of curva- 

 ture is remarkably constant. 



Looking now at the sexual variation displayed by B. bison, and 

 referring again to the excellent table of measurements given by Dr. 

 Allen, 6 we find that the variations within the female sex are very 

 similar to those of the male, but that the average for all measurements 

 is considerably less, and that in some measurements the range of varia- 

 tion in male and female does not overlap. It should be borne in mind, 

 however, that the difference between the sexes is emphasized by the 

 fact that all of Allen's female specimens were under ten years of age. 

 Comparing the average of the measurements of all the males with the 

 average of all the females, we find the latter differing from the former 

 as follows : size of skull, 22 per cent less, with no overlap of measure- 

 ments; length of horn cores, 23 per cent less; basal circumference of 

 horn cores, 40 per cent less, with no overlap ; circumference of horn 

 cores relative to their length along upper curvature, 110 to 145 per cent 

 in males, 81 to 105 per cent in females. No difference can be found 

 between the sexes in the direction of curvature of the horns, the range 

 of variation being about the same in both. 



From this it is evident that in B. bison the female differs from 

 the male in having a skull about 20 per cent smaller, and in possessing 

 more slender horn-cores. This statement is corroborated by the obser- 

 vation of Rhoads 7 on the herd of bison belonging to the Philadelphia 

 Zoological Society, containing six females and fourteen males. He 

 points out that the chief sexual difference is in the smaller basal caliber 

 of the horns of the females, the variation in curvature and angle of 

 growth not being greater than that found within a single sex, while 

 their length, due to the relatively longer sheaths, is greater Avith respect 

 to the size of the body in the females, the cores, however, averaging 

 shorter, and being more distinctly cylindrical throughout. 



«Op. cit. 



i Rhoads, S. N., Proc. Acad. Nat. Sei. Phila., vol. 49, pp. 483-501, pi. 12, 1897. 



