1150 



THE UNGULATES, OR HOOFED MAMMALS 



lobes. The group is widely distinguished from the Amblypoda by the structure of 

 the cheek-teeth, and not improbably by the number of digits having been three in 

 place of five. It is, however, decidedly the most generalized of the three South- 

 American extinct suborders, as is specially shown by the flattened astragalus. The 

 remarkable similarity of the molars of Astrapotherium to those of rhinoceroses must 

 probably be considered as largely due to parallelism, since the structure of the ankle 

 in the allied Homalodontotherium indicates that the group diverged from the common 

 ancestor before the modern Odd-Toed Ungulates had required their characteristic 

 foot structure. 



In the homalodontothere, representing the first family, the teeth, as shown in 

 the illustration on p. 1149, comprise the full number of z'f, c\, p\, raf, and have 

 no gap; the canines being rooted, and of relatively-small size, and the molars with 

 comparatively-short crowns. The upper premolars are nearly as complex as the 



I.OWER JAW OF THE ASTRAPOTHERE. 



(Much reduced.) 



molars; and the third upper molar is not very markedly different from the two 

 preceding teeth. The lower molars are in the form of double crescents, of which 

 the anterior develops a loop like that found in the horses. It is stated that the toes 

 terminated in claws. The one known species of the genus was an animal of the 

 approximate size of the Sumatran rhinoceros. 



The gigantic astrapothere, which alone represents the second family, differs 

 from the last genus by the more specialized and reduced dentition, the enlarged 

 teeth of each jaw taking the form of permanently-growing tusks, which are worn in 

 nearly the same manner as those of the pigs. The molars are more distinctly rhi- 

 nocerotic in structure, those of the upper jaw having taller crowns than those of the 

 homalodontothere, with a large posterior valley, and a well-developed projection in 

 the middle valley. The last of the series has the same triangular form as in the 

 majority of species of rhinoceroses; while the premolars are simpler than the molars. 



