76 MR. E. T. BENNETT ON THE GENUS OCTODON. 
ment of the Rodentia. The Herbivorous Rodents exhibit more strikingly perhaps than 
those of the Omnivorous tribe, characteristic forms in the configuration of the crowns 
of their molar teeth; and the form peculiar to each species or genus among them is 
not liable, as in the other tribe, to vary with the progress of life, and with the conse- 
quent extent of detrition which is occasioned by their continued use. The Herbivorous 
Rodents are, indeed, rodents par excellence ; for in them a provision has been made for 
the perpetual renewal of the rasping surface of all the teeth. In the Rodents generally 
the incisors are always growing from their base forwards ; and a never-failing succession 
of cutting or penetrating edges is thus ensured to them at their tips: their points, in 
constant process of wearing, are in equally constant process of protrusion. In the Her- 
bivorous tribe a similar arrangement prevails as regards the molars also. Destitute of 
true roots, and growing, like the incisors, from an enduring pulp, their crowns, although 
perpetually wearing away by the grinding and rasping actions which they exert upon 
the food and upon each other, are never destroyed: as their upper surface is rubbed off 
the deficiency is supplied from below, and by the continual growth from the base the 
requisite length of the tooth is maintained, while the crown is always preserved of the 
due height for mastication and furnished with those ridges and folds of enamel which 
were originally bestowed upon it, and which are perpetually renewed. The action of 
these teeth is rather that of rasping than of crushing, and it is in this manner that the 
food is reduced in the mouth to that state of minute subdivision which is essential to 
the animals that are provided with teeth of this description. In the animals of the 
Herbivorous tribe the curious structure of the fauces, originally described by Mr. Morgan 
in the Capybara, Hydrocherus Capybara, Erxl., and since observed in other Rodents, 
appears to be most developed: and it is only one among the numerous and beautiful 
illustrations of the adaptation of various portions of the organization of an animal to 
each other, to find in combination with a narrowing of the entrance of the pharynx 
to such an extent as to allow of the passage through it of none but the most minutely 
subdivided particles, a structure of teeth by which the existence of the means of so 
minutely subdividing the food should be permanently secured at all periods of the 
animal’s existence. 
At the time when Octodon was first made known to science it was remarked, as one 
of its most distinguishing peculiarities, that the form of the crowns of its molar teeth 
were, in the two jaws, strikingly dissimilar. But it was not at that time anticipated 
that the two forms of dentition exhibited by Mr. Cuming’s Rodent would each be found 
to be characteristic of another nearly allied genus. It is in this manner that Octodon 
becomes evidently intermediate between Ctenomys and Poephagomys, by having the 
molars of its upper jaw constructed on the type of those of the former, and the molars 
of its lower jaw on that of the latter genus. The relations of these several groups, as 
indicated by their dentary characters, will best be understood by a brief consideration 
