273 



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 ennnence 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 Xesodon imbricatus ; the second, of the size of a Zebra, is the 

 Nesodon SuUvani] the species to vrhich belong the portions of skull, 

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

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

 species of the size of a Rhinoceros, Xesodon niagnus, 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, PerissodactyJa 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 tlie 

 great natural group of TJng ul at a „hMt to indicate that the known mo- 

 difications of the skeleton of the extinct Toxodons and Xesodcns 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 Tododou, and 

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

 genus in the mammalian class. 



The paper is illustrated by twenty-three highly-finished draw- 

 ings, by Dinkel, of the fossil bones and teeth of the different species 

 of Nesodon. 



January 20, 1853. 

 J. P. GASSIOT, Esq., V.P. in the Chair. 

 The following papers were read : — 



1. " On the Extension of the value of the ratio of the Circum- 



