82 MOSTLY MAMMALS 



would, however, show that while the middle toe is not 

 markedly larger than either of the others, the bones of the 

 wrist are arranged on the linear plan, while in the ankle 

 the upper surface of the huckle-bone is nearly flat, or 

 intermediate between that of the elephants and the odd- 

 toed ungulates. Omitting mention of certain other minor 

 peculiarities in the structure of the limbs, if we now turn 

 our attention to the teeth, we shall see that these also 

 present features unknown in any living ungulates. We 

 find, for instance, in the first place, that the upper jaw is 

 furnished with two pairs of permanently growing chisel- 

 like teeth, comparable to the single pair of incisors in the 

 rodents or gnawing mammals ; these being opposed by 

 three pairs of nearly similar, although horizontally placed, 

 lower teeth. Such permanently growing incisor-teeth are 

 paralleled among existing ungulates in the hyrax, but the 

 toxodon stands alone in the order from the circumstance 

 that the cheek-teeth likewise grow throughout life, instead 

 of forming roots. Here, then, we have another point of 

 resemblance in the toxodon to the rodent order. When 

 we examine the form of the grinding surface of these 

 cheek-teeth, there does not appear any marked resemblance 

 to those of any existing ungulates. The link is, however, 

 furnished by certain allied forms from the older Ter- 

 tiary beds of Patagonia, known by the name of Nesodon, 

 of which the first fragmentary remains were brought to 

 Europe by Darwin, in the Beagle ; the toxodon being 

 confined to the Pampean deposits and the underlying beds 

 of Monte Hermoso. Now, in the nesodons, the structure 

 of the cheek-teeth clearly approximates to that character- 

 ising the odd-toed ungulates, although belonging to what 

 naturalists term a more specialised type. It is further 

 noteworthy that in these nesodons, although the cheek- 



