The great extent and activity of the pro cesses of dental development required for the preparation of the large and complex true molar teeth would seem to exhaust the power, which, in ordinary Pachyderms, is expended in developing the vertical successors of the deci duous teeth. In the primeval Mastodons above cited, this normal exercise of the reproductive force was not, however, wholly exhausted, and one premolar, of more simple form than its deciduous predecessor, was developed on each side of both jaws. But even this trace of ad herence to the archetypal dentition is lost in the more modified Proboscidians of the pre sent day.
Another and very interesting mark of adhe sion to the archetype is shown by the deve lopment of two incisors in the lower jaw in the young of some of the Mastodons, by the retention and development of one of these inferior tusks in the male of the Mastodon giganteus of North America, and by the reten tion of both in the European Mastodon longi rostris. No trace of these inferior homotypes of the great premaxillary tusks have been detected in the fcetus or young of the existing elephants.
In the gigantic Binotherium the upper in cisors were suppressed, and the two lower incisors were developed into huge tusks, which curved down from the symphysis of the massive urder jaw. Most of the grinders had two transverse ridges on the crown, as in the Tapir ; two deciduous molars, if not three, were succeeded vertically by two premolars, the second of which (p. 4 of the typical series) closely resembles the true molars, as in other Perissodactyles.
The typical dentition is departed from in the existing Hippopotamus by the early loss of p. 1, and the reduction of the incisors to 22 in both jaws : in the extinct Hippopota 2-2 mus of India p. 1 was longer retained, and the incisors were in normal number whence the term Hexaprolodon proposed for this inter esting restoration by its discoverers, Cautley and Falconer.
I have before remarked that the even-toed or artiodactyle Ungulates superadd the cha racters of simplified form and diminished size to the more important and constant one of vertical succession in their premolars. These teeth in the Ruminants, e. g. ( fig. 580., VII., Mose it us, p. 2, 3, 4), represent only the moiety of the true molars,or one of the two semi-e.ylin drical lobes of which those teeth consist, with at most a rudiment of the second lobe, as Cuvier very accurately describes*, and F.
Cuvier figures in pl. 94. of his useful work, " Dents des Mammiferes." An analogous morphological character of the premolars will be found to distinguish them in the dentition of the genus Sus (figured in my " pl. 140.,figs. I and 2), in the Hippopotamus (ib. pl. 143.), and in the Phacoeharus (lb. pl. 141.), where the premolar series is greatly reduced in number : yet this instance of a natural affinity manifested in so many other parts of the organisation of the artiodacty le genera has been overlooked in F. Cuvier's work above cited, although it is expressly designed to show how such zoological relations are illus trated by the teeth. Confiding in the accuracy of the Baron Cuvier's division of the hoofed quadrupeds into " Pachyderms " and "Rumi nants," M. F. Cuvier separates the non•ru minant Artiodactyles from the ruminant ge nera of the same natural division by interposing the Tapir, Hyrax, Rhinoceros and Elephant ; whilst the horse, which, in the size and com plexity of its premolars, as well as in many other characters, agrees closely with the other perissodactyle Pachyderms, is placed in close juxtaposition with the Ruininants.t Most of the deciduous molars of the Rumi nants resemble in form the true molars ; the last, e. g. (fig. 593., d. 4), has three lobes in the ower jaw like the last true molar (wt. 3.). They but there is a rudiment of d. 1 in the embryo fallow-deer, and in one of the most ancient of the extinct Ruminants (Doreatherium, Kaup) the normal number of premolars was fully de veloped.
Sufficient, it is hoped, has been adduced to prove that the molar series of the Diphyo donts is naturally divisible into only two groups, premolars and molars ; that the is ty 4--1 pical number of these s 4_4, 3-3 ; and that each individual tooth may be determined and symbolised throughout the series, as is shown in the instances tinder cut 580. If anything were wanting to prove the artificial character of a three-fold division of these teeth, and the futility of any other classification than that founded upon development, it would be af forded by the attempt to determine the homo logous teeth which is exemplified by the dotted line which traverses the series, and which crosses the teeth distinguished by the name " principales" in the great " Osteo graphie and Odontographie " now in course of publication by Prof. de Blainville.