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THE AMEEICAIT EISOXS. 



49 



limbS; of the ribs^ the dorsal spines, etc., are of frequent occurrence. As such 

 variations are now so well known to characterize vertebrates in general, 



each species having a considerable normal range of osteological variatiouj 

 they may be passed over without further remark. 



Amonf^- more unusual variations are the occasional development of an 

 extra rib, or an extra pair of ribs, which may articulate either with the last 

 cervical or the first lumbar vertebra. A famous instance of the latter was 

 presented by a specimen described by Cuvier (the first skeleton of the Amer- 

 ican bison that came under the eye of an osteologist), which had fifteen pairs 

 of ribs, and only four, instead of five, lumbar vertebrae (see above, p. 2). The 

 mistake to which this abnormal specimen gave rise in respect to the number 

 of dorsal and lumbar vertebras and the number of pairs of ribs possessed 

 by the American bison as compared with the aurochs, has already been 

 noticed, -™- a mistake that still survives in some of our leading text-books of 

 comparative anatomy. In the Museum of Comparative Zoology is a male 

 from Kansas possessing a supplemental pair of ribs which articulate with the 

 last cervical vertebra, instead of with the first lumbar, as in the case of 

 Cnvier's specimen. 



Variations in the form of the skull are often strikingly apparent, affecting 

 not so much, however, the relative size of the different parts, or the pro- 

 portion of width to length, as the frontal outline or profile, and the curvature 

 and relative direction of the horns. In respect to the profile, the frontal 

 region varies in different specimens of the same sex and of corresponding 

 ages in the forehead being either flat, or even slightly concave, or very con- 



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vex (see Plates V, VI, and VII). The horns are usually so much depressed 

 that when the skull is placed on a flat surface with the dorsal aspect down- 



+ 



Avard the points will not touch the surface on which the skull rests, — -in 

 other words, do not rise to the plane of the forehead; in other specimens 



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they sometimes rise so high as to prevent the skull from touching the flat 

 surface by a space of one or two inches. The horn-cores are also sometimes 

 directed backward far beyond the plane of the occiput, though usually not 

 reaching it (see Plates V, VI, and VII). Such differences as these are so con- 

 siderable that they are sometimes, in allied groups, regarded as indicative of 



specific differences. 



. The variation in lentrth in a series of a dozen ao-ed male skulls rano-es from 



500 to 600 mm., but the usual range of variation is between 500 and 550 

 mm. The extremes in breadth are 240 and 280 mm., ranging usually be- 



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