2-2 2-2 tn. = 21; 1-1' 2-2' 2-2the additional teeth being i. 3 and M. 2 (c, fig. 594.; in which the demonstration of the true deciduous character of d. 4 and d. 3 is shown by the germ of their vertical successor, p. 4, which is exposed in the substance of the jaw). The next stage is the shedding of d. 3, and the acquisition of in. 3 (D, fig. 594.). Then d. 4 is shed by the ascent of p. 4 into its place (E, fig. 591.). Afterwards in. 4 is acquired, and, in the MG ervits gigas, p. 4 is simultaneously pushed out (F, fig. 594.).
Thus, four individuals of this species may be found to have the same number of molars, .4 i.e. ; two of these may seem, on a cur sory comparison, to have them of the same shape, e. g., like c and E, fig. 594 ; or like D and F. 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 he present, as at p. 4, c and D, or at nr. 4, The result of such examination may be to show that not one of the four kangaroos with 44 the in. had the same or homologous 44 teeth. The four grinders, e. g., may be d. 3, d. 4, in. na. 2 ; as in c : or, d. 4, in. 1, in. 2, »z. 3 ; as in n : or, p. 4, in. 1, in. 2, in. 3 ; as in E : or, M. 1, 2, m. 3, and in. 4 ; as in F.
But the change does not stop here : as age advances, in. 1 is shed, and the molar series is reduced numerically to the condition of a ; but, instead of in. I, d. 4, and d. 3, it consists of m. 2, in. 3, tn. 4.
Finally, tn. 2 is shed, and the dentition is re duced to the same numerical state as at A, fig. 594.: the teeth, however, being m. 3 and m. 4.
i The order here described is not precisely that which is followed in some of the smaller species of kangaroo. In llfaeropus e.g., the acquisition of in. 3 is not accom panied by the shedding of d. 3, but the molar series is numerically 5-5 so, likewise, in 5-5 " this species, the acquisition of nr, 4. is not accompanied by the displacement of p. 4 ; 55 and a molar series of is long retained ; 5-5 but, at the earlier period cited, the teeth are d. 3, d. 4, in. 1, m. 2, and in. 4
and, at the lacer period, they are: p. 4, m. 1, M. 2, ni. 3, and in. 4.
These symbols, 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. Had those facts been described in the ordinary way, by means of verbal phrases or definitions of the teeth, e. second deciduous molar, representing the fourth in the typical den tition," instead of d, 4, and so on,--the de scription would have occupied much more space, and have levied such a tax upon the attention and memory, as must have tended to enfeeble the judgment, and impair the power of seizing and appreciating the results of the comparisons.
Each year's experience strengthens my conviction that the rapid mid successful pro gress of the knowledge of animal struc tures, and of the generalisations deducible therefrom, will be mainly influenced by the determination of the nature or homology of the parts, and by the concomitant power of condensing the propositions relating to them, and of attaching to them signs or sym bols, equivalent to their single substantive names. In my work on the " Archetype of the Skeleton," I have denoted most of the bones by simple numerals, which, if generally adopted, might take the place of names; and all the propositions respecting the centrum of the occipital vertebra might be predicated of as intelligibly as of " basioccipital." The symbols of the teeth are fewer, are easily understood and remembered, render unnecessary the endless repetition of the ver bal definition of the parts, harmonize con flicting synonyms, serve as a universal language, and express the author's meaning in the fewest and clearest terms. The entomologist has long found the advantage of such signs as a and , signifying male and female, and the like ; and it is time that the anatomist should avail himself of this powerful instrument of thought, instruction, and discovery, from which the chemist, the astronomer, and the mathematician have obtained such important results.
(I1. Owen.)