III.] 



ASTRAPOTHERIA. 



8l 



Astrapotheria. 



curious parallelism in the development of this group to that of the 

 horses. 



A second sub-order of ungulates, entirely confined to the Santa 

 Cruz and Patagonian beds of Patagonia, is repre- 

 sented by the two generic types known as Astrapo- 

 therium and Homalodontotherium, each of which constitutes a 

 family by itself. Rivalling the rhinoceros in bulk, both of these 

 extraordinary creatures differ from the last group by the structure 

 of their molar teeth, which approximate to those of the Rhino- 

 cerotidce, those of the upper jaw having a continuous outer wall, not 

 divided into lobes. In the ankle-joint the astragalus differs from 

 that of the preceding group in having its upper surface flat, thus 

 resembling that of the elephants. In both the wrist and ankle 



FlG. 15. GRINDING SURFACE OF RIGHT UPPER MOLAR OF Astrapotheiium. 



the component bones were arranged on the linear plan, and not 

 improbably each foot had five toes. The vertebrae of the neck 

 were short, with flat terminal faces ; and in correlation with this 

 shortness of the neck, the limbs and feet were more or less 

 abbreviated. In the first-named of the two genera each jaw was 

 provided with an enormous pair of tusks, wearing obliquely against 

 L. 6 



