OCR Text |
Show 1900.] STRUCTURE OF THE MUSK-OX. 703 can also be seen iu the ontogenetic development of the Musk-ox. In the calf these processes are quite flattened from the sides as in Bos and many Antelopes, which perhaps is the primitive shape, as it is found in so many different forms. Later, in adult animals, the processes become thickened in such a way that they become triangular in section through the development of a lateral crista whereby a broad posterior face is produced. Distally the processes become compressed in an antero-posterior direction and end iu a transverse edge. These modifications are of course effected for the purpose of getting larger space for the insertion of the muscles, and I do not lay much weight upon them: I mention them only to show that the processus paroccipitalis of the Musk-ox does not much resemble those of the Sheep, which are posteriorly narrow, slender and tapering towards the tips. These processes of the Musk-ox are slightly curved towards the median line l, although not so much so as in the Sheep, Oxen, Buffaloes, and many other ruminants. They cannot be termed small (cf. Biitimeyer), as their length in an adult bull measures about 3 cm., which is as much as ^ of the height of tbe occipital region from the lower surface of the condyles to the sutura lambdoidea. The same relation is already found in the calf (resp. 2 cm. and 8 cm.). " The basi-occipital bone in Ovibos moschatus is quadrate in outline," Boyd Dawkins says (I. c). In the young calf it is quite hexagonal, but with age the posterior sides are shortened so that the corpus of the bone becomes quadrate in outline; and therein lies of course a certain similarity with the Sheep, but also with several Antelopes. It seems, therefore, contrary to Dawkins's opinion, questionable whether this characteristic can be of any other value than that of separating these animals from the Oxen, which have the basioccipital bone differently shaped. Dawkins also points out the difference between Bos and Ovibos with regard to the anterior muscular impressions of this bone, the former having them " supported on a tuberosity." Such things are of little importance; but as the question is open to discussion, it mav be mentioned that in Sheep and Goats these impressions are situated at the sides, partly on the lateral surface, of the bone, and separated by a smooth area nearly as broad as the bone. In the Musk-ox, on the contrary, the same impressions cover the ventral surface of the bone so that they nearly meet in the median line in the adult male, but do not extend laterally (fig. 7). The basioccipital of the Musk-ox is traversed by a low median keel which is already indicated in the calf. Bichardson's exact description (I. c. p. 69) of these parts need not be repeated. The auditory bulla is subjected to considerable ontogenetic changes, as can be seen from the following measurements. In the calf it is rather large and inflated. Its length is 39 and its greater width 20 mm. In the adult cow it also looks inflated, 1 This may be subject to variation, as Richardson says that they " descend straight," but Riitimeyer found them " einwartsbogen wie beim Argali." 4 6* |