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I 
THE AMERICAN BISONS. 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 variation, — 
they may be passed over without further remark. 
Among 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 vertebrae 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 Zodlogy is a male 
from Kansas possessing a supplemental pair of ribs which articulate with the 
last cervical vertébra, instead of with the first lumbar, as in the case of 
Cuvier’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- 
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- 
ward 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 
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 length in a series of a dozen aged male skulls ranges 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- 
