| OCR Text |
Show 1893.] DR. C. J. FORSYTH MAJOR ON MIOCENE SQUIRRELS. 201 form by the addition of the so-called heel or talon to the original trigon. The name of heel or talon is borrowed from the teeth of Carnivora, where this part, as the name implies, generally appears in a reduced form compared with the rest of the tooth. In other orders the so-called talon is, as a rule, a well-developed part of the inferior molar, so that it seems a priori highly improbable to consider as a later development that part which, in the majority of Mammalia, constitutes the whole posterior moiety of the lower molars. What we at present know of the oldest forms of Perissodactyla, Condylarthra, Rodentia, and even of some forms of Creodonta, as revealed especially by the Cernaysian fauna of Reims1, does not in the least justify a similar assumption. On the contrary, the " talon," far from showing a tendency to disappear, is in several of these archaic Eutheria very prominent, and even more distinctly developed than in any later form, not only in longitudinal extension, but partly even in the elevation of the cusps, as compared with those of the anterior moiety. The question of the heel leads us to an objection made by Fleischmann, who on the whole accepts the tritubercular theory, but maintains that the cusps of upper molars are not directly homologous to those of the lower molars; or, in other words, that not only does the internal side of upper molars correspond to the external side of lower molars, as admitted by Cope and Osborn, but that, besides, the anterior part of upper corresponds to the posterior part of lower molars2. In reply to Fleischmann, Osborn states that " this objection would be fatal to a uniform system of nomenclature for the upper and kmer cusps if it could be sustained," but that it is disproved by a comprehensive survey of the Mesozoic trituberculates, especially of the Amblotheriidae aud Spalacotheriidte3. Neither Osborn nor Fleischmann seem to be aware that, if the latter is right, his objection will be fatal not only to the homology of upper and lower cusps, but also to the theory, for the primitive trigon which includes the protocouid, the supposed homologue of the reptilian cone, would in that case be found to be formed for the greater part by the very heel which the theory considers to be a late addition. There can be no doubt as to the correctness of Fleischmann's statement, which is easy to verify. A left upper anterior milk-tooth of Bidelphys, for instance, is at first sight very difficult to distinguish from one of the right lower series. Even in such specialized molars as those of modern Ruminants, in holding side by side a right upper and a left lower molar, or vice versa, what appear to be the mutual homologies are to be traced out even to 1 Lemoine, "Etude d'ensemble sur les dents des Mammiferes fossiles des environs de Reims," Bull. Soc. Geol. de France, iiie serie, t. xix. 1891, pp. 263-289, pis. x., xi. 2 A. Fleischmann, " Die Grundform der Backzahne bei Saugethieren und die Homologie der einzelnen Hocker " (Sitzungsber. d. k. Preuss. Akad. d. Wiss. Berlin, 1891). 3 Osborn and Wortman, I. c. pp. 84. 85. |
