MOLAR TEETH OF THE MAMMALIA EDUOABILIA. 263 



We may look on the Artiodadyla — even-toed or cloven-footed 

 mammals — as one of the most homogeneous groups in the class, 

 not only in respect to the structure of the extremities, but also in 

 that of the cranium, vertebrae, etc. But here we have both Buno- 

 dont and Selenodont types of molars. The Perissodactyla, as de- 

 fined by the feet, axis, palate, etc., present us with the Symhoro- 

 dont, PalcBotheriodont, Tapirodont, and Selenodont types of denti- 

 tion in the superior series, and the Hippodont, Hyracodont, Pa- 

 IcBotheriodont, and Trichecliodont types in the mandibular teeth. 

 The pentadactyle, plantigrade type, for which I have used the 

 name Prohoscidia, presents us with the Tricliecliodont, Batlimo- 

 dont, and Loxolophodont types of molar structure. Among Siren- 

 ians, as defined by the marked peculiarities of the entire skeleton, 

 we have the great differences in dentition presented by Halicore 

 and Trichecus, the former being Haplodont, the other Trichecho- 

 dont. Finally, the Marsupial group is unquestionably well defined, 

 and here Phascolomys is PtycJiodont ; Didelphys, Bunodont ; Pe- 

 taurus between Bunodont and Selenodont, and the Kangaroos and 

 their gigantic extinct allies the DiprotodontidcB, Tricliecliodont. 



It is thus evident that the molar types are everywhere subor- 

 dinated to those which we call ordinal ; therefore in the case of 

 the placental mammals, and especially those with complex folding 

 of the cerebral hemispheres, to the types of construction of the 

 feet. As to the modifications presented by the canine and incisor 

 teeth, these exist within a still more narrow range of variation ; 

 for instance, in the allied genera Equus, Rliinaster, and Symboro- 

 don ; Sus and Pliacoclicerus ; Batlimodon and Unitatlierium, and 

 others. 



It is thus probable that modifications in the three points of 

 structure considered were introduced in the following order : 



First, Of the feet. 



Second, Of molar type.* 



Third, of the relations of canines and incisors. 



"With regard to the significance of the tnree types of feet. Pro- 

 boscidian, Perissodactyle, and Artiodactyle, it has been already 

 remarked : " It is to be observed that the lines of Ungulata, 



* In the case of the Marsupialia the relation of the dental and extreniital types 

 may be reversed. Thus we have pentadactyle plantigrade forms (Opossums) and 

 (nearly) didactyle difritigrade forms (3/ac>-o/)Ms) in the same order. Aho Ilahnatu- 

 rm and DiproUxhn, both Trichcchodonts, differ in the type of feet, as do the car- 

 nivorous Didclphjs and 7'hi/lacimcs, both Bunodonls. 



