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University of California Publications in Geology [Vol. 9 



21440. Complete ramus, all teeth present. Middle-aged. Eancho La Brea. 

 21181. Complete ramus, young adult. Eancho La Brea. 



21441. Complete ramus, Dm2 and Dm3 still in, but nearly pushed out. Eancho 



La Brea. Figs. 12a and 12b. 



21438. Incomplete ramus, P~ and Ps unworn. Eancho La Brea. 



21439. Incomplete ramus. Dm- still in place. Eancho La Brea. 



21435. Incomplete ramus, Ms unworn, Ms not yet appeared. Eancho La Brea. 

 21180. Complete ramus. Milk dentition complete. Eancho La Brea. 



21437. Front part of ramus. Milk dentition complete, only Mi in place. 

 Eancho La Brea. 



21436. Complete ramus, Dms unworn. Eancho La Brea. 



CEANIUM 



An examination of the measurements of all the specimens of B. 

 antiquus considered in Table I, execept 21183 and 21188, which are 

 excluded on account of their youth, shows that in none of the meas- 

 urements of the cranium is there more than 20 per cent variation, 

 and in some of the measurements considerably less than that. It is 

 strange that the measurements of the facial region are throughout far 

 more constant than those of the cranial region. For instance, in the 

 basinasal length there is a variation of over 16 per cent, and in the dis- 

 tance from the foramen magnum to the rear of the hard palate, about 

 20 per cent, while in none of the facial measurements does the variation 

 exceed 10 per cent. The same tendency for greater variation in the 

 cranial portion of the skull is shown by the table of measurements 

 given by Allen for B. bison, but is not as marked. 



An examination of the measurements shows at once that the speci- 

 mens fall into two natural groups, one with small, slender, horn-cores, 

 the other with large, robust horn-cores. By analogy with the existing 

 species, we may safely assume that the former are females, and the 

 latter males. While there may be slightly smaller average measure- 

 ments of the skull proper in the females than in the males, the differ- 

 ence is inappreciable. In the horn-cores, however, the difference is 

 striking. From tip to tip of the horn-cores there is a variation of 

 about 10 per cent in the males, and 12 per cent in the females, but there 

 is a. difference of over 20 per cent in the average measurements of the 

 two sexes, with no overlap. Looking now at the measurements of single 

 horn-cores along the upper curvature, we find a still greater difference, 

 in fact, over 25 per cent, since the difference in extent of horns is due 

 entirely to the length of the horn-cores, the width of the forehead being 

 fairly similar in both sexes. In circumference at the base of the horn- 



