Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/449

Rh UNGULATA.] have greatly puzzled all zoologists who have attempted to unravel them. Macrauchenia, an animal with a Camel-like neck, is now known to be a Perissodactyle, though in some characters somewhat aberrant. The articulation of the fibula with the calcaneum is an Artiodactyle or perhaps generalized character. The teeth ally it to Falseothcrium and Rhinoceros. Homalodontothcrium, from the banks of the river Gallegos, south-east Patagonia, is known by the teeth alone, which, though very generalized, are on the whole rhinocerotic. Ncsodon, from the same locality, also only known by the dentition and some parts of the skull, connects the last and Macrauchenia with Toxodon. These three genera have the typical dental formula of i f, c 1, p |, m f = 44. Toxodon is an animal about the size of a Hippopotamus ; it was first discovered by Darwin, and many specimens have since been found in Pleistocene deposits near Buenos Ayres, and described by Owen, Gervais, and Burmeister. The teeth consist of large incisors, very small lower canines, and strongly curved molars, all with persistent roots, the formula being apparently i |, c, p |, m f = 38. The cranial characters ex hibit a combination of those found in both Perissodactyles and Artiodactyles, but the form of the hinder part of the palate and the absence of an alispheuoid canal belong to the latter ; and the tym panic, firmly fixed in between the squamosal and the exoccipital, ankylosed to both, and forming the floor of a long upward-directed meatus auditorius, is so exactly like that of the Suina that it is difficult to believe it does not indicate some real affinity to that group. These characters seem to outweigh in importance those by which some zoologists have linked Toxodon to the Peris- sodadyla, and the absence of the third trochaiiter and the arti culation of the fibula with the calcaneum tell in the same direction. The structure of the feet is not completely known, but Cope has shown that the tarsal bones differ altogether from those of either Artiodactyles or Perissodactyles, and more nearly resemble those of the Proboscidea than any other known Ungulates. Mesotherium, also called Typotherium, from the same locality, was an animal rather larger than a Capybaua, and of much the same general appearance. Its skeleton is completely known, and shows a singular combination of characters, resembling Tuxodoii or a I lG 106. Cranium and Lower Jaw of Mesotherium cristatum. J nat. size. From Gervais. generalized Ungulate on the one hand, and the Rodents, especially the Leporidae, on the other. In the presence of clavicles it differs from all known Ungulates, and in having four lower incisors from all Rodents. The teeth are i, c %, p f , m f = 24. It will thus be seen that, although our knowledge of many of these forms is still very limited, we may trace among them a curious chain of affinities, which would seem to unite the Ungulates on the one hand with the Rodents on the other ; but further materials are required before we can establish with certainty so important a rela tionship, one which, if true, would alter materially some of the prevailing views upon the classification of mammals. UNGULATA VEEA. la the typical Ungulata the feet are never plantigrade, and the functional toes do not exceed four, the inner digit being suppressed, at all events in all forms which have 427 existed since the Early Eocene period. The os rnagnum of the carpus articulates freely with the scaphoid. The allantois is largely developed, and the placenta, so far as is known, is uon-deciduate, the chorionic villi being either evenly dilfused or collected in groups or cotyledons (in Pecora). The testes descend into a scrotum. There is never an os penis. The uterus is bicornuate. The mammae are usually few and in guinal, or may be numerous and abdominal (as iuSuina), but are never solely pectoral. The cerebral hemispheres in exist ing Ungulates are well convoluted. The group is now, and has been throughout the whole of the Tertiary period, composed of two perfectly distinct sections, differing from each other, not only in the obvious characters of the structure of the limbs, but in so many other parts of their organization that they must be con sidered as of the rank at least of suborders. The characters of these divisions, first indicated by Cuvier, were thoroughly established by Owen, by whom the names whereby they are now generally known were proposed. SUBORDER PERISSODACTYLA. This is a perfectly well-defined group of Ungulate mammals, represented in the actual fauna of the world by only three distinct types or families the Tapirs, the Rhinoceroses, and the Horses poor in genera and species, and (except in the case of the two domesticated species of Equus, which have been largely multiplied and diffused by man s agency) not generally numerous in individuals, though widely scattered over the earth s surface. Palaeon- tological records show very clearly that these are but the surviving fragments of a very extensive and much varied assemblage of animals which flourished upon the earth throughout the whole of the Tertiary geological period, and which, if it could be reconstructed in its entirety, would not only show members filling up structurally the intervals between the existing apparently isolated forms, but would show several marked lines of specialization which have become extinct without leaving any direct successors. The following are the principal characters distinguishing them from the Artiodactyla. Premolarand molar teeth in A n c FIG. 107. Bones of Fore Foot of existing Peiissodactyles. A, TYpir (Tapirus indicus), x; B, Rhinoceros (Rhinoceros sumatrensis), Xj; C, Horse (Eqwu caballus), x. U, ulna; R, radius; c, cuneiform; I, lunar; s, scaphoid ;, unci- f orm ; m, magnum ; td, trapezoid ; tm, trapezium. Otteolcgy of Mammalia. continuous series, with massive, quadrate, transversely ridged or complex crowns, the posterior premolars resem bling the true molars in size and structure. Crown of the