d 5, and only becomes a permanent molar because there is no premolar developed above it, so we may regard the tooth marked m 1 in figs. 221-230 as being an antecedent tooth of the deciduous series, rendered permanent by a like reason, the suppression, viz. of p 4. In other words, that m 1 in fig. 227 is the homologue of d 4 in fig. 294, and that the true homologue of p 4 is not developed in the Marsupialia.

The homologies of the teeth of the Kangaroo are illustrated in fig. 296, according to this idea of them; the dental formula of both the Macropodide and Hypsiprymnidæ being

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.

In the deciduous dentition of the great Kangaroo (Macropus major) the canines are rudimental, and are absorbed rather than shed. No other of the deciduous series is calcified, save the molars d 2 and d 3, fig. 296, unless the permanent incisors be developed and retained milk-teeth. When the young animal finally quits the pouch the dentition is

the upper incisors being i 1, the molars d 2 and d 3 of the typical dentition. This stage is exemplified in the lower jaw at A (fig. 296). The next stage shows the acquisition of i 2 in the upper jaw, and d 4 in both jaws, and the formula is

the additional teeth being i 3 and m 1 (ib. c), in which the demonstration of the true deciduous character of d 2 and d 3 is shown by the germ of their vertical successor p 3, which is exposed in the substance of the jaw. The next stage is the shedding of d 2, and the acquisition of m 2 (ib. D). Then d 3 is shed by

the ascent of p 3 into its place (ib. E). Afterwards m 3 is acquired; and in the Macropus gigas, p 3, simultaneously pushed out (ib. F).

Thus, four individuals of this species may be found to have the same number of molars, i. e. 4:4; two of these individuals may seem, on a cursory comparison, to have them of the same shape, e. g., as in C and E, or as in D and F, fig. 296. In fact, to determine the identity or difference in such instances, it requires that the substance of the jaws be examined, to see if the germs of successional teeth are present, as at p 3, C and D, or at m 3, E. The result of such examination may be to show that not one of the four Kangaroos with the m 4:4 had the same or homologous teeth.

The four grinders, e. g. may be-d 2, d 3, d 4, m 1 ; as in C; or d 3, d 4, m 1, m 2 ; as in D; or p 3, d 4, m 1, m 2; as in E; or d 4, m 1, m 2, and m 3; as in F.

The changes, however, do not end here. As age advances, d4 is shed, and the molar series is reduced numerically to the condition of B; but, instead of d 2, d 3, and d 4, it consists of m 1, m 2, m 3.

Finally, m 1 is shed, and the dentition is reduced to the same numerical state as at A; the teeth, however, being m 2 and m 3.

The symbols used, it is hoped, are so plain and simple as to have formed no obstacle to the full and easy comprehension of the facts explained by means of them. If these facts, in the manifold diversities of Mammalian dentition, were to be described in the ordinary way, by verbal definitions, e. g., the second deciduous molar representing the third in the typical dentition,' instead of d 3, and so on, the description of dental development would continue to occupy much unnecessary space, and would levy such a tax upon the attention and memory as must tend to enfeeble the judgment and impair the power of seizing and appreciating the results of the comparison.

Each year's experience has strengthened the writer's conviction that the rapid and successful progress of the knowledge of animal structures, and of the generalisations deducible therefrom, will be mainly influenced by the determination of the homology of parts and organs, and by the concomitant power of condensing the propositions relating to them, and of attaching to them signs or symbols equivalent to their single substantive names. In the writer's Works, CXL, CXLI, CXLIV, he has denoted most of the bones by simple numerals. The symbols of the teeth are fewer

in number, are easily understood and remembered, and, if generally adopted, might take the place of names. They would then render unnecessary the repetition of phrases, harmonise conflicting synonyms, serve as a universal language, and at the same time express the expositor's meaning in the fewest and clearest terms. The entomologist has long found the advantage of such signs as and, in reference to the sexes of insects, and the like; and it is hoped that the time is now come when the anatomist may avail himself of this powerful instrument of thought, instruction, and discovery, from which the chemist, the astronomer, and the geometrician have obtained such important results.

CHAPTER XXX.

ALIMENTARY CANAL AND APPENDAGES OF MAMMALS.

§ 223. Mouth.-Fleshy lips form the main characteristic of the mammalian mouth. But they are wanting in the Monotremes, with other significant shortcomings of mammalian excellence. Lips are, here, transitorily manifested, it is true, at the suckling period; but soon degenerate into the pergameneous border of the beak in Platypus, and are reduced, in Echidna, to the scarcely movable margin of the small terminal oral orifice of the adult. The Cetacea show the greatest extremes within the limits of a natural group in the development of the lips. They are barely represented in the Porpoise, fig. 297, and other Delphinide by

297

Section of mouth and nose, Porpoise.

the low, firm, ridge of integument, supported by adipo-fibrous tissue with scarce a trace of orbicularis oris': while in the Whale (Balana) the upper lip falls down like a thick curtain some feet in depth concealing the baleen, and overlapping the