508 Zoological Society. 



incisors, which are fitted for cutting roots*. The auditory builce 

 are of moderate size, but rather smaller than in Octodon ; the lower 

 jaw is larger and much stronger than in the two genera mentioned. 

 In these characters the present animal makes so near an approach 

 to Ctenomys, that it might with propriety be placed in that genus 

 were it not that in the structure of the teeth there exists a difference 

 fully as great as that which gave rise to the generic distinction of the 

 little groups of which the family Octodontidce is composed. In Octo- 

 don, Poephagomys and Ctenomys, the enamel of the molar teeth enters, 

 in the form of a fold on each side, into the body of the tooth, but the 

 folds from opposite sides do not meet. In the Rodent which forms 

 the subject of these observations, the crown of each molar is divided 

 into two parts by the meeting of the folds of enamel of the outer and 

 inner side, and the surface of these teeth may be compared to a 

 series of cylinders (two to each tooth), which are much compressed 

 in the antero-posterior direction. The three foremost molars in 

 each jaw are equal in size, and the posterior molar is smaller than 

 the rest. In position, the last molar of the upper jaw differs from 

 the others, being as it were twisted, so that the two transverse lobes 

 are placed obliquely. 



With our present very limited knowledge of the small Rodents of 

 the southern parts of South America, and especially of the western 

 coast, it is impossible to judge of the value of such a modification of 

 the molar teeth as is here pointed out. It is possible that the species 

 of Octodontidce may vary more or less among themselves in the 

 structure of these teeth, in which case the so-called genera, esta- 

 blished as the species are discovered, will require a revision, as do 

 very many of the genera of Rodentia ; in the mean time, however, 

 it is necessary that sectional names should be imposed on such 

 species as will not agree tolerably well with the definitions of the 

 genera published as such. Agreeably to these views, the subgeneric 

 title Schizodon\ is proposed for the present new Rodent. The prin- 

 cipal external characters may be thus expressed : — 



ScHizoDOif Fuscus. Schiz. supvu. griseo-fuscus, subths obscure flavo 

 tinctus : pedibus pilis obscure fuscis tectis ; auribus mediocribus ; 

 Cauda, fused quoad longitudinem caput fere aqnante, pilis brevis- 

 sims tectd. 



unc. lin. 

 Longitudo ab apice rostrl usque ad caudse basin .... 9 



Cauda 1 8 



tarsi digitorumque 1 5^ 



auris 5^ 



* The burrowing Sciurides and Miiridce, as compared with the typical 

 examples of their respective groups, present a corresponding modification of 

 the skull ; the strength of the cranium is greater, and the cranial cavity 

 smaller. It is difficult to estimate the amount of intelligence in these ani- 

 mals, but, judging from the size of the brain, it would appear that there 

 existed an intimate connexion between the food of the animal and its in- 

 telligence ; the food in one case leading the animal to habits which cause 

 it to be exposed to numerous dangers which do not occur in the other. 



t From ffX'C^j divido, et doov;, dens. 



