OCR Text |
Show 590 MR. M. F. WOODWARD O N [May 5 (6) The evidence advanced in support of the tritubercular theory is insufficient to prove that the upper molars primarily evolved on the lines of that theory. (7) Owing to want of material, trituberculists have been led to assume that the upper molars of the early Mammalia passed through similar stages to those which they have determined for the lower teeth, and consequently they have in most cases incorrectly identified the primary cone (save in Peralestes and the living Centetidae and Chrysochloris). (8) That as regards the primary cone, its ontogeny recapitulates its phylogeny. I do not mean to deny for one moment the occurrence of the tritubercular type of upper molar tooth, nor even to underrate its phylogenetic importance ; for no one who has studied cusp ontogeny can fail to notice the frequency of its appearance, and the fact that often (though not always) the three cones of the trigon are the first to appear during development. "What I desire to point out is, that there is no evidence to show that this type of upper molar arose in the way suggested by trituberculists, and that they have in most cases overlooked the true primary cone. If the triconodont tooth be a stage in the evolution of the mammalian molar, then I should believe that the anterior cone disappeared, the main cone becoming enlarged as the paracone and the posterior one as the metacone. At this stage the upper teeth overhang and bite outside the lower molars, and the future antero-internal cone (protocone) was developed as an internal shelf acting as a mortar for the cusps of the lower teeth, and at a much later period developed a cusp. The hypocone arose in a similar way with the elongation of the teeth. The function and origin of the external cingulum with its numerous cusps (2-4) is difficult to understand, for in the living Mole it is quite outside and free from all contact with the lower molars ; possibly it is of use to insect-feeding animals, giving them greater hold of their slippery prey. In the Centetidae and Peralestes, the upper molars could not have overhung the lower ones to the same extent, consequently no internal lobe bearing the protocone was developed and the external cingulum was very largely developed. I have purposely left out all reference to the multituberculate and concrescence theories, having restricted my researches to endeavouring to ascertain whether the trituberculate theory respecting the upper molars rested upon any solid basis, and whether one of the molar cusps was more primitive in its mode of origin than the others. Ontogenetically, I have failed to find any support for the concrescence theory, neither do I consider that any of the evidence put forward by Rose and Kiikenthal is at all conclusive in its favour. On comparing the several famdies which grouped together |