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Show 1893.] DR. C. J. FORSYTH MAJOR ON MIOCENE SQUIRRELS. 197 Riitimeyer starts from the assumption that the primitive type of Mammalian molars had a conical or cylindrical shape (" homa;- odont" type), which simple form became complex in course of time, so that we must expect to find a more simple type of molars the more we recede in time. Riitimeyer's views were supported by the fact that, in several of the oldest deposits then known, of Tertiary Mammalia were met with abundantly the Lophiodontida>, showing the zygodont molar in its typical form. Kowalevsky1 held the same views as Riitimeyer and pointed out, besides, that a less simply constructed form of molars is met with in the older Tertiary, especially amongst "primitive Ungulates" (e. g. " Microchcerus")2. But whilst he did not enter upon the possible relations between such complex sextubercular forms and the zygodont or lophodont type (as it was called later), Cope had urged already, in 1874, that a bunodont tooth was the ancestral form of the modern placental molar, thus tacitly admitting that the zygodont molar is a secondary, a derived form3. The various modifications of ungulate molars were traced back by Cope to a quadri tubercular type, and somewhat later he traced the sectorial type of inferior molars to a quinquetubercular or tuberculosectorial type4. The discovery of the Puerco, the oldest known Tertiary M a m malian fauna of America, gave opportunity for the recognition by Cope of a still more primitive type of superior molar, the tritubercular type, the great majority of the Puerco Mammals having, according to Cope, their superior molars constructed after this type5. In the latest review of the Puerco fauna it is stated that almost all the Placentalia show the tritubercular type in their superior molars, as, out of 82 Placentalia, only four are quadrituberculate. The quinquetuberculate or tuberculosectorial type of inferior molars is equally widely spread, although less generally so, 64 out of 82 Placentalia possessing it6. The farther development of the tritubercular theory in these last years is treated of at length in all the recent Manuals, as the whole phylogeny of the Mammalia is directly connected with the question. Not one palaeontologist who has dealt with the argument has 1 W. Kowalevsky, " Monographie der Gattung Anthracotherium, Cuv.," Palaeontographica, xxii. 1873, 1874, pp. 210, 263, 264. 2 " Je tiefer wir in die Schichten dringen, je altere Formen wir finden, desto complicirtere Gestalten tauchen imnier auf. . . . ; also kann das als em Wink dienen, wie weit wir noch von der primitiven Form des Zahnes sind" (I, c. p. 230, note 1). 3 E. D. Cope, " On the Homologies and Origin of the Types of Molar Teeth of Mammalia Educabilia," Journal Academy Nat. Sciences of Philadelphia, new series, vol. viii. part 1 (Philadelphia, April 1874), pp. 71-89. 4 L. c. and E. D. Cope, " On the Trituberculate Type of Molar Tooth in the Mammalia," Pal. Bulletin, no. 97, Proc. Amer. Philos. Soc, Dec. 7, 1883 (publ. Jan. 2, 1884), p. 326. 3 " On the Trituberculate Type &c," I. c. G E. D. Cope, " Synopsis of the Vertebrate Fauna of the Puerco Series," Transact. American Philos. Soc, Aug. 1888, p. 299. |
