PROFESSOR OWEN ON SOME SPECIES OF THE EXTINCT GENUS NESODON. 299 
there are, also, two longitudinal risings along the outer side of the tooth. The chief 
differences presented by the Toxodon are seen in the simplicity of the enamel-folds, 
the low and equal development of the two external longitudinal ribs or risings, the 
relatively narrower grinding surface, and the more evident interruption of the ex- 
ternal coat of enamel at the anterior, posterior, and internal parts of the tooth. 
Moreover, there is no trace of canines or of outer incisors {i 3) in the adult Toxodon, 
and the anterior upper incisors are proportionally very long and large ; but the dif- 
ference might be less in the permanent upper incisors of the Nesodon. 
The correspondence between the teeth of the lower jaw of the Toxodon and 
Nesodon is of the same instructive character. No quadruped, recent or extinct, 
presents so close a resemblance to the lower molars of the Toxodon, in regard to 
their narrowness, and the length and deep implantation of the straight undivided 
crown, as does the Nesodon, espeeially in the first true molar, exposed in the section 
shown in fig. 7, ni 1. The (most probably) deciduous incisors of the same specimen 
deviate, however, considerably from the peculiar disposition, size, shape and curva- 
ture of the six lower ineisors in the adult Toxodon platensis, and this difference 
becomes of more importance as illustrative of the generic distinction of the two 
animals, from the proportions and arrangement of the permanent incisors and canines 
in the lower jaw of the adult Nesodon, first noticed in the ‘ Reports of the British 
Assoeiation’ for 1846 (Transactions of the Sections, p. 66), where the affinity of the 
Nesodon to the Toxodon is indicated, and the interval between the latter genus and 
Macrauchenia is stated to be “ partly filled by the newly-discqvered Nesodon." 
We may, now, therefore, regard the following as the generic characters of the 
Nesodon : — 
Dental formula,—/ |^, c g, m |E-3=44. The teeth subequal, and in an 
unbroken series. Incisors trenchant, with long, slightly curved crowns, but not 
disproportionately large, and of limited growth, being implanted by a tapering 
fang: Canines smaller, and not exceeding in length the contiguous premolars: 
Molars, above, 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 : the enamel is deposited in a thin layer. 
Ihe lower molars with long and straight, transversely compressed crowns, divided 
by an external longitudinal indent into two unequal lobes, both penetrated at the 
inner side by a fold of enamel, which is complex in the hinder lobe. 
The bony palate entire and extending 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 protuberant post-glenoid process ; a strong and deep 
zygoma; the orbit and temporal fossa widely intercommunicating; premaxillaries 
joining the nasals. 
MDCCCLIII, 9 R 
