Ungulata

The foot structure of the condylarths was likewise of interest. In the five-toed hind foot the ankle bones were much like those of a contemporary creodont, with a ball-and-socket joint at the lower end, an arrangement to which the name Condylarthra refers. In the fore foot of Phenacodus the two rows of carpal bones were placed directly above each other, "like unstruck bricks," whereas in typical ungulates a more displaced, or alternating arrangement, was the rule. E. D. Cope supposed that Phenacodus was in this respect also ancestral to later ungulates but Matthew showed that in both Phenacodus and the later ungulates the contrasting ar rangements of the carpal elements above mentioned were prob ably both derived independently from the still older interlocking creodont type.

Amblypoda

(q.v.).—In the basal Eocene of New Mexico occur the fragmentary remains of a diversified series of small hoofed mammals collectively known as Taligrada, or primitive Amblypoda. Some of the smaller genera were hardly bigger than insectivores and with teeth suggestive of insectivore-creodont ancestry, while the larger taligrades (Periptychus, Pantolambda) were about the size and proportions of stocky badgers. The upper molar teeth in the smaller forms had three main cusps arranged in a triangle with the apex toward the inner side ; the small inter mediate cusps or conules were more or less circular; the main in ternal cusp was flanked by small cusps borne by the anterior and posterior ridges or cingula. In Periptychus the upper molar cusps were all subcircular and the enamel surface was pleated. The brain-cast indicates a brain of extremely primitive form with large olfactory lobes and a minimum development of the neopal lium. In Pantolambda the upper molar crowns bore two sharp outer V's and a centrally placed conical internal cusp.

In all the taligrades the feet were short, especially so in Panto lambda. This feature becomes greatly emphasized in the later members of the order, the corysphodons and Dinocerata. Cory phodon is characteristic of the Lower Eocene of Wyoming and England. It was broad-headed and almost hippopotamus-like. Re lated forms have recently been found in the Eocene of Mongolia.

Uintatherium, Dinoceras and their allies were characteristic of the later Eocene. Their bodies were gigantic, larger than modern rhinoceroses, with massive, post-like limbs and extremely short stubby toes. They had sabre-like upper tusks, six or more horn like bony outgrowths on top of the skull and a brain of low type. Notoungulata.—The condylarths and taligrades disappeared from the fossil record of the northern hemisphere in early Eocene, but there is some reason to believe that some of them reached South America and there gave rise to the amazingly varied series of herbivorous mammals which are often referred to collectively as notoungulates. These flourished for millions of years in Pata gonia and adjacent regions, while the perissodactyls, artiodactyls and other ungulates held sway in the northern world. These notoungulates exemplified the law of adaptive radiation on a grand scale. Protected by geographical barriers from the deadly

competition of their larger-brained northern analogues, they ex ploited all the economic possibilities for ungulates available in South America and gave rise to the wide diversity of forms that covered the ancient pampas. Some (the protypotheres) were small and swift-running like rabbits; others (the smaller litop terns) were like slender-limbed three-toed horses; some (Mac rauchenia) paralleled the llamas and camels; others closely paral leled the rhinoceroses, while a few almost rivalled the elephants in bulk (Pyrotherium).

Litopterna (q.v.).—Among the most primitive of the entire series of notoungulates is the genus Didolodus, a small forerunner of the litopterns known only from the upper cheek-teeth, which in some ways recall those of the smaller taligrades. In the swif t footed proterotheres of the Miocene of Patagonia both cheek-teeth and feet suggest those of the Oligocene three-toed horses of the northern world. But in the upper molars the main inner cusp is central rather than anterior in position and the posterior cross crests fail to meet the middle point of junction of the two main outer cusps, as they do in the three-toed horses, while the feet, although superficially horse-like, differ profoundly from the horses in the detailed arrangements and contacts of the tarsal elements. The larger litopterns (Theosodon, Macrauchenia), while in gen eral appearance paralleling llamas and camels, yet are more nearly related to the horse-like proterotheres in their deeper characters.

Entelonychia, or Homalodontotheria.—In the "Notostylops beds" of Lower Oligocene or Upper Eocene in Patagonia occur a strange group of ungulates, varying in size from a rabbit to a rhi noceros. Of these the most famous form, Notostylops, had a small skull in which the upper grinding teeth have oblique and flattened outer walls remotely suggesting those of recent rhinoceros. The lower molars bore long obliquely-placed blades on the hinder part or talonid. The skull as a whole suggests that of the recent Hyrax. In certain early members in which the feet are known there are five digits on the fore and hind feet, the terminal bones are cleft for the attachment of fairly large hoofs, the main ankle bone (astragalus) is more or less flattened. A single very small jaw, the genus Arctostylops of Matthew, found in the Lower Eocene of Wyoming, appears to be related to these South American Ente lonychia. Another apparently related type is found in the Eocene of Mongolia. These small jaws seem to increase the probability that the South American Entelonychia, like other members of the notoungulate series, had been derived from some basal Eocene or Upper Cretaceous forerunners of the taligrades and condylarths of the northern world. Of the later Entelonychia the best known is the Santa Cruzian (Lower Miocene) genus Homalodontotherium, a larger form in which all the teeth had become high-crowned and all the incisors, canines, premolars and molars were pressed to gether in a continuous series without break or interval.