RUMINANTIA.
Of other forms of beasts subsisting on the vegetable pro ductions of the earth, and more akin to actual European Her bivore, there co-existed, in Europe, with the now exotic genera Elephas, Rhinoceros, Hippopotamus, etc., a vast as semblage of species, nearly all of which have passed away. The quadrupeds called "Ruminants," from the characteristic second mastication of the partly-digested food by the act called " rumination" or " chewing the cud," constitute at the present period a circumscribed group of Mammalia, which Cuvier believed to be "the most natural and best-defined order of the class.' He characterized it as having incisive teeth only in the lower jaw (fig. 128, c), which were replaced in the upper jaw by a callous gum. Between the incisors and molars is a diastema, in which, in certain genera only, may be found one or two canines. The molars (fig. 128), h, almost always six on each side of both jaws, have their crown marked by two double crescents, with the convexity turned inwards in the upper set, outwards in the lower. The four legs are termi nated by two toes and two hoofs, flattened at the contiguous sides, so as to look like a single hoof cloven ; whence the name " cloven-footed," also given to these animals. The perfect cir cumscription and definition of this order, so desirable by the systematic zoologist, is indeed invaded, in the actual Rumi nantict, by certain peculiarities of the camel tribe.
In entering upon the evidences of the first appearance in this planet of the order of animals, which now are the most valuable to man, it may be well to call to mind the characters of the Anoplotherium. The upper true molars have two double crescents, convex inwards, one of the inner ones being encroached on by a large tubercle, the reduced homologue of which may be seen in the internal inner space of the crescents in the ox and some other Ruminants. The lower true molars also, at one stage of attrition, form crescentic islands of enamel, with the convexity turned outwards, as in Ruminants, the last molar having the accessory crescent behind. The functional
hoofs were two in number on each foot, but must have resem bled those of the camel tribe in shape ; the scaphoid and cuboid of the tarsus were distinct also, as in the Camelicke ; and the metacarpal and metatarsal bones were divided, as in the water musk-deer (Moschus aquaticus), and in the embryos of all Ruminants. The dentition of the extinct Dichodon (figs. 102, 103) made a still nearer approach to that of the Rumi nants. The chief distinction of this and other extinct Herbi vores with double crescentic molars is the completion of the upper series of teeth by well-developed incisors. But the pre maxillaries in the new-born camel contain each three incisors, one of which becomes fully developed. The Camelidce are horn less, like the Anoplotherioids and Dichodonts ; and with one exception—the giraffe—all Ruminants are born without horns.
Thus the Anoplotherium, in several important characters, resembled the embryo Riimins Tit, but retained throughout life those marks of adhesion to a more generalized mammalian type. The more modified or specialized form of hoofed animal, with cloven feet and ruminating stomach, appears at a later period in the tertiary series.
The modification of the upper molars of the existing Rumi nant quadrupeds consists in the lower and less pointed lobes of the crown, the unworn summits of which are at first rather trenchant, like curved blades, than piercing. They are soon abraded by mastica tion, and present the crescentic lobes of dentin*, b,c,c0 shown in fig. 124. The transverse double-crescentic valley (g, contains a thicker layer of cement, and forms two detached crescents in worn teeth. The premolars resemble in structure one half of the true molars.