This author abandons the classification of the molar series adopted by the Cuviers, without assigning his objections to it ; and proposes another, in which he divides the series into " avant-molaires, principales; wad arriere-molaires;" he exemplifies this division by the human dentition, in which the five grinders on each side of both jaws are for mulised as " deux avant-molaires, une princi pale, et deux arriere-molaires." • With regard to the characters of these kinds of teeth, the avant-niolaires are "simple or complex," the principale is " trenchant," and the arriere-molaires are " tuberculous. "1 But as shape is not a constant character, especially in the "principale," the author pro poses another from its position, describing it as " being implanted below the root of the zygornatic process of the maxidary bone " in the tipper jaw ; and stating that the tooth which opposes it below, and is in advance of it, or crosses in front of it, is the lower " principale." In defining the dentition of the genus Fells t, M. de Blainville accordingly assigns 1 "avant molaire," 1 " principale," and 2 " arriere molaires" in the upper jaw ; and 1 " avant molaire," 1 " principale," and 1 " arriere-ino hare " in the lower jaw. In another part of the same work §, he, however, proposes another formula, viz., 2 " avant-molaires," 1 " princi pale," and 1 " arriere-molaire " above ; 1 " avant-molaire," 1 " principale," and 1 " ar riere-molaire " below ; but, taking either of these determinations, or the dental formulae which he assigns to other carnivorous genera, and comparing them with his formula of the molar series in the Quadrumana and Man, we find that a tooth which displaces and succeeds a milk-tooth in one species is made the homo logue of a tooth, which, in Man and Quadru mana, rises above the gum without displacing any predecessor: in other words, the " prin. cipale " is a premolar in certain genera, and a true molar in other genera. I may refer to my Paper on the Classification of the Molar Teeth in the " Annalcs des Sciences,"* and to the concluding pages of the chapter on the teeth of the Carmvora in my" Odontography " (p. 514), in proof that a " molaire principale " does not exist in nature ; that the characters by which it is defined by M. de Blainville are artificial ; and that they fail in their application to determine the teeth in the series of pla cental Mammalia with deciduous and perma nent teeth.
In the series of figures, fig. 580., the continuous line traverses the tooth or its homologue, in Man and the Ruminant, which Clavier distinguished as the " rnolaire carnas siere :" the dotted line traverses that tooth which M. de Blainville distinguishes as the " molaire principale " the letters and numbers symbolise the teeth and indicate their indi vidual homologies, and the binary division of the molar series, which it has been one object of the present Article to illustrate. I shall conclude it by showing how these symbols may be applied to the exposition of facts in the comparative anatomy of the teeth, and for that purpose select the complex and intricate subject of the succession of the teeth in the kangaroo.
The chief modifications of the marsupial dentition are described and illustrated in the article MARSUPIALIA (Vol.
When that volume was published I had not had the means of tracing the period and order of the development and succession of the entire series of teeth in any of the marsupial genera. The first of the five grinding teeth on each side of the jaws of the wombat had shown, by its displacing a milk-tooth vertically, that it was a premolar ; and the adjoining molar, by its earlier development and use, was plainly the first of the four true molar teeth. In the carnivorous and insectivorous families, the marked difference of form and size of the last four teeth from those intervening between them and the canines, had also induced me to class them as true molars, although I had not got the developmental evidence of the fact, except in the case of the Kangaroos and Potoroos (21facropus and Mypsiplymnus). The analogy, however, seemed to be sufficient to justify the generalisation that the Marsupial differed from the Placental Diphyodont mam mals in having four true molars, i. e., 3— instead of ' and also that they differed 3 3-3 in having only three premolars, i. e. p.
44 instead of — p. ; the typical number of the 77 grinding being the same. The genus Mynnecobius offered the most able exception here, as the Manatee had done in the placental series, in the increased number 8-8 9-9 of the grinding teeth, e. g. to or which, according to the shape of the crowns, were divided, in the Myrnzecohius, into p.
nt. — • but the order of development 3-3' 6-6 ' and succession may show that the number of premolars is greater, and that of the true molars less. The probably marsupial Thyla cotherium or Amphitherium from the Oxford oolites — the most ancient of all known mam mals—had as many as twelve teeth in each molar series, besides a canine and three incisors, and by their form I have grouped 3-3 1-1 6-6 6-6 them as: p. m.
c. 1— t' 6-6' An interesting field of observation still re mains open in regard to the period and order of developementof the deciduous and permanent teeth, in the different carnivorous, omnivorous, insectivorous, and frugivorous Marsupials. At present I have ascertained the required facts only in the herbivorous family (Poephaga).
The permanent dental formula of both the 3-3 Macropodidee and Hypsiprymnidce is, i.
1-1 1-1 4-4 in. 30. The canines, which are confined to the upper jaw, are small or minute when retained ; and disappear after being represented " en germe" in most of the true kangaroos.
The deciduous dentition of the great Kan garoo (Macropus major) is, i.
3-3 1=-1' c. 1-1 0=0' 2-2 The canines are rudimental, and are absorbed rather than shed. The deciduous incisors are shed before the young animal finally quits the pouch : when this takes place, the dentition is :— ' i. = 12 ; 1-1 2-2the upper incisors being i. 1, the molars d. 3 and d. 4 of the typical dentition. This stage is exemplified in the lower jaw at A, fig. 594. The next stage shows the acquisition of i. 2 in the upper jaw, and in. 1 in both jaws, and the formula is: i.
2-2 22 1-1 — — clan. — nz. = 18. (a,fig.59.1..) 1-1' 2-2' 1-1 At one year old, the dentition is: i.