150 



EVOLUTION OF MAMMALIAN MOLAR TEETH 



clearly seen on the buccal or external side of the tooth, which in 

 some specimens have large and well defined alveoli. Two morpho- 

 logically older but geologically more recent stages are figured by 

 Forsyth Major (Fig. 114). Taken in connection with the fact that 

 the crowns of these teeth show a vestigial triangular pattern and in 

 comparison with the molar of Sciuroptrrus xanthipcs (op. cit., PI. 39, 

 Fig. 15), and with the fact that the hypertrophy of the lingual 

 portion of the tooth is secondary, little doubt remains that we have 

 here a tooth with three originally subequal roots disposed in a 

 triangle, and a trigonal and probably tritubercnlar crown. 



Thus both divisions of the Bodentia, when studied from the ana- 

 tomical or comparative zoological standpoint, or from the standpoint 



A 



C 



FIG. 115. Evolution of hypselodont rootless crowns in the upper molars of American ancestors 

 of the Hares (Leporidse), showing (1) the disappearance of the external roots of the tooth, (2) the 

 limiting of the enamel to a broad band on the inner side. From Matthew. 



A. Paltvolayus brachyodun. Titanotherinm Beds, Lower Oligocene. 



B. Palcuolagus turgidus (less worn). Oreodon Beds, Middle Oligocene. 



C. Palceolagus turgidus (more worn). Oreodon Beds. 



D. Palceolagus inter medius. Leptauchenia Beds, Upper Oligocene, the crown is now rootless, 

 the enamel is becoming confined to the anterior face. 



E. Lepus americanus. Recent. The enamel is now confined to the anterior face. xf. 



of the milk teeth, or from the standpoint of palaeontology, apparently 

 lead back to a tritubercular, trigonal pattern of the crown. As among 

 all the other divisions of the mammalia we confidently predict the 

 absolute demonstration by palaeontology of the derivation of the Eodentia 

 from trituberculate ancestors. On the other hand, Dr. Wortman l has 

 shown that in certain ancestral Eocene Sciuromorphs (Paramys) in 

 the conrse of molarization of the inferior premolars, the apex of the 

 original single-pointed premolar remains in the antero-internal cusp 

 and not the antero-external cusp of 2h P> whereas in the Ungulata, 

 Carnivora, Insectivora, Primates, and probably other orders, the primi- 

 tive tip or true protoconid is in the antero-external cusp. Hence, if 

 the true molar cusps are homologous with similarly placed cusps in 

 the premolars (see pp. 195-200) the so-called protoconid or antero- 

 external cusp in the molars of brachyodont Rodents is not homologous 

 with the similarly placed cusp of other orders. Dr. Wortman also 



1 Amer. Jour. Sci., Vol. XV., June, 1903, pp. '216--218. 



