Thus the class Mammalia, in regard to the times of formation and the succession of the teeth, may be divided into two groups : — the " Monophyodonts,"t or those that generate a single set of teeth ; and the " Di phyodonts,"$ or those that generate two sets of teeth.
The Monophyodonts include the orders Monotrenzata, Bruta (Edentata, Cuv.), and Cetacea (Cetacea Vera, Cuv .): all the rest of the order are Diphyodonts. In these, the first set of teeth are called the milk or de ciduous teeth : the second set, the adult or permanent teeth ; although the teeth of this set are for the most part, like those of the first set, of limited growth, contracting to a root or roots, and being shed in greater or less proportion during the life-time of the species ; which life-time, in wild Carnivora and Herbivora, is dependent on, and would seem, indeed, to be determined by, the dura tion of the adult teeth.
The particulars of the Monophyodont dentition will he found under the Articles MONOTREMATA, Vol. III. p. 387.; CETACEA, Vol. I. pp. 563. 571. 573. ; (see my Odonto graphy, p. 345. pls. 87-91.); and EDEN TATA, Vol. II. p.53.; (see also Odontography, p.317. pls. Examples of some of the striking modifications of dental structure pre sented by recent or extinct animals of the order Brute, are given in figs. 548. and 574. of the present article. It will be observed that I have qualified the generalisation as regards the Monophyodont character of the Cetacea, by citing only that part of Cuvier's order which he termed " true or carnivorous Ce tacea." The animals of the order Sirenia has been called in question.* I have, how ever, discovered in specimens of the Malayan (herbivorous Cetacea of envier) differ in many organic particulars from the Cetacea proper*, and in none, perhaps, more strikingly than in having both deciduous and permanent teeth; this succession takes place, at least, with regard to the upper incisors of the Du gong, fig. 575.
These teeth project from the gum in the male sex ;, but neither upper nor lower in cisors are visible in the female.f The supe rior incisors are but two in number, in both sexes ; in the male, they are moderately long, subtriedral, slightly and equally curved, of the same diameter from the base, and deeply excavated to near the apex, which is obliquely bevelled off to a sharp edge, like the scalpriform teeth of the Rodentia. The
form and extent of the persistent pulp-cavity of this tooth are shown in the figure of its longitudinal section, in my " Odontography," pl. 93. fig. 4.; it becomes longer and more pointed than in the permanent incisor of the younger male (fig. 575, i). When fully de veloped, only the extremity of this tusk pro jects from the jaw, at least seven-eighths of its extent being lodged in the socket, the parietes of which are entire ; and the exterior of the great premaxillary bones presents an unbroken surface. In the female Dugong, the growth of the permanent incisive tusks of the upper jaw is arrested before they cut the gum, and they remain through life concealed in the premaxillaries ; the tusk is solid, is about an inch shorter and less bent than that of the male ; it is also irregularly cylin drical, longitudinally indented, and it gradually diminishes to an obtuse rugged point ; the base is suddenly expanded, bent obliquely outwards, and presents a shallow excavation. These were conjectured by Home to be the " milk-tusks ;" they are, however, character istic of sex, not of age ; and the existence of deciduous tusks at any period in the Dugong Dugong which I have dissected at the Zoological Society, the true deciduous in cisors of the upper jaw (fig. 575, d i) co existing with the permanent ones (i). They are much smaller than the permanent tusks of the female, and are loosely inserted by one extremity in conical sockets immediately an terior to those of the permanent tusks, ad hering by their opposite ends to the thick tegumentary gum, which presented no out ward indication of their presence.
When this gum was stripped off the bone, the deciduous tusks came away with it ; and this may account for their usual absence in dried crania of immature Dugongs, in which, nevertheless, their alveoli are generally suffi ciently conspicuous. True permanent in cisors are not developed in the lower jaw of the Dugong ; those which are occasionally found there are abortive remnants of the first or deciduous series, which are not destined at any time to rise above the gum (fig. 575, d i 3.).