212 Royal Society, 



American herbivores are shown to agree with each other in characters 

 of greater value, derived from the osseous and dental systems, than 

 any of those by which the Nesodon resembles either the Perissodac- 

 tyle or Artiodactyle divisions of hoofed animals. 



The genus Nesodon is characterized by the following modifications 

 of the teeth, which in number and kind are according to the typical 

 dental formula above given. Incisors trenchant, with long, slightly 

 curved crowns, of limited growth : canines small, not exceeding in 

 length the contiguous premolars. Molars, in the upper jaw, with 

 long, curved, transversely compressed crowns, which contract as they 

 penetrate the bone and ultimately develope fangs ; the outer side of 

 the crown ridged, the inner side penetrated by two more or less 

 complex folds of enamel, leaving insular patches on the worn crown : 

 enamel thin. The lower molars, long, straight, and compressed ; 

 divided by an external longitudinal indent into two unequal lobes, 

 both penetrated at the inner side by a fold of enamel, which is com- 

 plex in the hinder lobe. All the teeth have exserted crowns of 

 equal height and arranged in an unbroken series. The bony palate 

 is entire and extends back beyond the molars, the maxillaries and 

 palatines forming the back part in equal proportions. A distinct 

 articular cavity and eminence for the lower jaw ; the eminence long 

 and concave transversely, short and convex longitudinally; a protu- 

 berant post-glenoid process ; a strong and deep zygoma, the orbit 

 and temporal fossa widely intercommunicating ; the premaxillaries 

 join the nasals. 



Of the genus presenting the above dental and osteal characters 

 the author defines four species : — the first, about the size of a Llama, 

 is the Nesodon imbricatus ; the second, of the size of a Zebra, is the 

 Nesodon Sulivani ; the species to which belong the portions of skull, 

 with the teeth, described in the present memoir, did not exceed the 

 size of a large sheep, and is termed the Nesodon ovinus ; fourthly, a 

 species of the size of a Rhinoceros, Nesodon magnus, is satisfactorily 

 indicated by a grinder of the upper jaw. In conclusion, the author 

 remarks, that the osteological characters defining the orders of hoofed 

 quadrupeds, called Proboscidia, Perissodactyla and Artiodactyla, are 

 associated with modifications of the soft parts of such importance, 

 as not only to establish the principle of that ternary division of the 

 great natural group of Ungulata, but to indicate that the known mo- 

 difications of the skeleton of the extinct Toxodons and Nesodons of 

 South America, in the degree in which they differ from the osteology 

 of the already defined orders of Ungulata^ must have been associated 

 with concomitant modifications of other parts of their structure which 

 would lead to their being placed in a distinct division, equal to the 

 Proboscidia ; and, like that order, to be more nearly allied to the Pe- 

 rissodactyla than the Artiodactyla. This new division of the Ungulata 

 the author proposes to call Toxodontia, and he remarks that its dental 

 and osteal characters, while they illustrate the close mutual affinities 

 between the Nesodons and Toxodons, tend to dissipate much of the 

 obscurity supposed to involve the true affinities of the Toxodon, and 

 to reconcile the conflicting opinions as to the proper position of that 

 genus in the mammalian class. 



