MACRAUCHENIA. 603 



quently, the left ramus of a lower jaw, found in tertiary deposits of 

 Buenos Ayres, has been received at the British Museum, containing 

 six molar teeth, three true and three false, most of which manifest the 

 same pattern as the single fossil tooth above adverted to, and more 

 decidedly and fully determining the resemblance to the lower molar 

 series of the Palseothere, as will be obvious by comparing the 

 tigure of the grinding surface of the Buenos Ayrian fossil teeth 

 (PL 135, fig. 7,) with those of the Pal^othere (ib. fig. 6). The 

 chief generic distinction is the absence of the third lobe in the last 

 molar of the South American Pachyderm, by which it more closely 

 resembles the Rhinoceros ; but it differs, hke the Palseothere, from 

 the Rhinoceros in the greater exterior convexity and the equal height 

 of the two demi-cylindrical lobes of which the last premolar and the 

 three true molars are composed : and it differs from both Paleeothere 

 and Rhinoceros in the more simple form of the second and third 

 premolars : the first small premolar of the South American Pachy- 

 derm may have existed in the part of the fossil jaw which is 

 mutilated, anterior to the second premolar in the British Museum 

 specimen: it is hypothetically added to the series, in PI. 13.5, fig. 7, 

 at p. 1. If, however, it was not functionally developed in the 

 Macrauchenia, this genus would again differ from the Palaeothere 

 and resemble the Lophiodon and Tapir, in the number of the 

 grinding teeth of the lower jaw. In the figure above cited, the 

 teeth are given of half their natural size : the enamel is smooth, 

 the dentine compact, and the coronal cement forms a very thin 

 layer, as in both Rhinoceros and Palaeothere. In the more simple 

 form of the second and third molars we may trace a slight approxi- 

 mation to the Anoplotherian and Ruminant types ; and, since no 

 other Pachyderm of the size required to correspond with that 

 indicated by the jaw and teeth here described, and, at the same 

 time, with close affinities to the Palseothere and the Cameloid 

 Ruminants, has, hitherto, been discovered in South America, 

 excepting the Macrauchenia, I deem it to be highly probable that 

 the teeth in question belong to that interesting annectant genus of 

 Anisodactyle Pachyderms. 



