1892.] NATURAL SCIENCES OF PHILADELPHIA. 411 



forms, at least in matters of detail. For example, no one would 

 infer from the ontogeny of the horse's molar, that it had been 

 derived from such types of dentition as those displayed by Hyra- 

 cotheriiim, Aiiehitherlam, etc., and yet if any phyletic series may be 

 regarded as well established, it is that of the Equidie. Knowing 

 the phylogeny, we are able to eliminate cenogenetic features and to 

 show that the ontogeny in no way contradicts the results of pal- 

 aeontology. 



(5) Fleischmann objects to Osborn's homologizing the cusps of 

 the upper and lower molars. Strictly speaking, of course, an 

 upper tooth is not homologous with the corresponding lower tooth, 

 and nevertheless we speak very properly of the upper canine as 

 equivalent to the lower, etc. In the Trlconodon molar there is no 

 reason why the anterior basal cusps of the upper molars should not 

 be homologous with the anterior cusps of the lower molar in the 

 same sense that the protocone is equivalent to the protoconid. All 

 that Osborn's nomenclature implies is that in the tritubercular 

 tooth a certain element represents the anterior cusps of the tricono- 

 dont molar, and in no way denies the possibility that the primitive 

 simple coues,(haplodont stage) are reversed in the two jaws. There 

 is not a particle of evidence, however, for such a reversal in the 

 haplodont stage, nor is any fact known which could suggest a 

 rotation of the upper and lower molars in opposite directions, in 

 stages subsequent to the triconodont. If the series of genera selected 

 by Osborn really represents the stages of molar development, there 

 is nothing like rotation, but merely a lateral displacement of the 

 cusps. 



(6) The conclusions which Fleischmann reaches from his exam- 

 ination of the marsupial dentition are quite inadmissible. Palseon- 

 tological discovery seems every day to make it more clear, that 

 the placental mammals are not derived from any marsupials as yet 

 known, but that placentals and marsupials form two distinct and 

 divergent series. The objection of uncertain phylogenetic relation- 

 ship which Fleischmann urges against Osborn's series of genera 

 applies with more force to any reasoning founded upon marsupial 

 dentition. In particular, Fleischmann protests against the view 

 that the talon of the lower molars is something superadded to the 

 primitive triangle, and unrepresented in the triconodont tooth. But 

 this objection leaves out of account the fact that the talon can be of 

 importance only when the upper and lower molar series begin to 



