312 LAND MAMMALS IN THE WESTERN HEMISPHERE 



The neck was of moderate length and the body, as indicated 

 by the long, arched ribs and the greatly expanded hip-bones, 

 was extremely bulky and massive. The spines of the anterior 

 dorsal vertebrae were excessively long, forming a great hump 

 at the withers. The limbs and feet were columnar, like those 

 of an elephant ; the feet were supported on a great pad, 

 while the hoofs were mere excrescences on the periphery of the 

 foot. The bones of the fore-arm were entirely separate and 

 the ulna was very stout ; in the lower leg also the bones were 

 not coossified, but the fibula was but moderately heavy. 

 This is a sharp contrast to the arrangement found in the horses 

 and in those hoofed animals generally which are swift runners 

 and have slender, elongate limbs and feet, such as deer, ante- 

 lopes, camels, etc. Heavy, slow-moving animals, like elephants, 

 tapirs, rhinoceroses, etc., almost always have separate fore- 

 arm and leg-bones and generally a heavy ulna. The number of 

 digits was four in the front foot and three in the hind. The 

 genera differed in the proportions of limbs and feet, one 

 having them longer and less ponderous than another, and, 

 no doubt, the former was of swifter gait. 



At a certain level in the White River beds the ftitanotheres 

 abruptly cease, disappearing with what seems like startling 

 suddenness. In all probability, however, the extinction was 

 more gradual and its apparent abruptness was due, partly at 

 least, to the break in the deposition of the beds, which is very 

 obvious. Such a break, or "unconformity," as geologists 

 call it, almost always implies an unrecorded lapse of time, which 

 may have been very long. However it came about, gradually 

 or suddenly, the extinction of these great animals is difficult 

 to explain ; no Carnivora of the time could have been formi- 

 dable enemies and they had no rivals in their own walk of life. 

 Their stupidity may have been a factor, but it seems more 

 likely that the onset of some new infectious disease, perhaps 

 imported by incoming migrants from the eastern hemisphere, 

 gave the coup de grace. In the lower substage, beneath the 



