| OCR Text |
Show 1896.] MAMMALIAN DENTITION. 585 cusp pbylogeny as advanced by the supporters of the Cope-Osborn tritubercular theory. This is a very striking and important fact, and one which will no doubt be considered by trituberculists as strongly supporting their theory, especially as it is generally stated that these trituberculate Insectivores most nearly, amongst living mammals, approach the Jurassic Trituberculata in the character of their molars. This statement is certainly true for the lower jaw, but can be hardly said to hold for the upper molars, there being no resemblance between the teeth of the upper jaw of Centetes, Ericulus, and Chrysochloris l and those of Peralestes, and only an apparent one with Kurtodon (Stylodon), for Osborn (16) himself states that this latter is not trituberculate but ridged2. Turning now to the first group and examining it in the light of the supposed primitive nature of the protocone, w e find here that the upper molar teeth are more complex, possessing 4 or 5 cusps, that the outer cusps (the para- and meta-cones) are more strongly developed than the inner ones; and in accordance with this w e find both these cusps developing before the protocone-an anomalous condition when we remember that the last-named cusp is supposed to be the primitive axis of the tooth, the remaining cusps being mere outgrowths from it. Perhaps, if these Insectivora were the only forms possessed of such a condition, w e might agree with Osborn (15) that this is merely a case of accelerated development; but they are not alone in this respect, for in M a n (19), in some Ungulates (22), and in certain polyprotodont Marsupials (20), the paracone invariably develops first, the protocone being either 2nd or 3rd in order of appearance. In fact, in every mammal so far examined, with the exception of the two Insectivores before mentioned, the paracone develops directly from the primitive dental germ and before either tbe protocone or metacone. The constancy of this condition is such that I do not think w e can pass over it so lightly as Osborn does, as may be seen from the following quotation (15. p. 503): "In fact the external cusps not only appear before the internal cusp, which palaeontology shows to be the more primitive, but they assume the crescentic form earlier. In other words, their development is accelerated." (Italics mine.) If the protocone represents the summit of the original protodont tooth of the ancestor of the Mammalia, it must be the direct continuation of the primitive dentinal germ, and as such should be found to develop in a line with the axis of that structure. That this is not the case is well seen in fig. 32 (Pl. X X V I . ) , where the paracone (5) is found to be identical with the primitive dentinal germ and the protocone (7) appears as a mere internal ledge growing out from Chrysochloris is trituberculo-sectorial, possessing a small heel, and not a pure trituberculate as usually stated. Lydekker (10) compares Peralestes and Chrysochloris, but I fail to see the resemblance. It is very difficult to ascertain Osborn's views regarding Kurtodon, for in his large memoir (16. p. 210) he states that there is no real homology between the Kurtodon and Chrysochloris dentition, whereas in hisadditional notes (16 (?) he appears to regard Kurtodon as one of the Trituberculata. PEOC. ZOOL. Soc-1896, No. XXXVIII. 38 |
