This series cannot be traced back of the Pleistocene, in which epoch it was not only far more widely distributed, but also very much more diversified, no less than three extinct genera, in addition to the existing one, having been found in the North American Pleistocene. One of these (_†Symbos_), which extended from Alaska to Arkansas, had horns which were smaller and shorter than in the modern genus, and, even when fully developed, did not meet in the middle line of the head. The other two genera, from California (_†Euceratherium_ and _†Preptoceras_ Fig. 116, p. 203), are of great interest as showing affinities to the Musk-Ox and also to sheep and to certain antelopes, such as the Takin (_Budorcas_) of northern India and Tibet. They serve to connect the musk-oxen with other Cavicornia, but the origin of all these animals is to be sought in Asia. In Recent North America there are four or five species of sheep (_Ovis_) which are confined to the mountainous and broken areas of the western part of the continent and extend from Alaska to Mexico. The “Bighorn” or Rocky Mountain Sheep (_Ovis canadensis_) is characterized by great, spirally coiled horns in the rams, in the ewes the horns are very much smaller and nearly straight; the other species differ but slightly from this type. The species _O. canadensis_ has been found in the Pleistocene, but nothing further is known of its history. Evidently, the sheep were late immigrants. “The geographical distribution of wild sheep is interesting. The immense mountain ranges of Central Asia, the Pamir and Thian Shan of Turkestan, may be looked on as the centre of their habitat.” “Sheep are essentially inhabitants of the high mountainous parts of the world, for dwelling among which their wonderful powers of climbing and leaping give them special advantages. No species frequent by choice either level deserts, open plains, dense forests or swamps. By far the greater number of species are inhabitants of the continent of Asia, one extending into North America [should read, four or five] one into Southern Europe and one into North Africa.... No remains that can be with certainty referred to the genus [_Ovis_] have been met with in the hitherto explored true Tertiary beds, which have yielded such abundant modifications of Antelopes and Deer.”[10]