86 



ON THE HOMOLOGIES AND ORIGIN OF THE 



The preceding table has been already published in its essential features in the 

 Keport on Geological Survey of the Territories, 1873, p. 648.* 



It remains now to ascertain whether the genealogical or taxonomic relations ex- 

 pressed by the teeth coincide with those derived from the other diagnostic regions of 

 the body. First of these must be selected, as of chief importance, the limbs and feet. 



We may look on the Artiodactyla— 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, vertebra, etc. But here we 



have both Bimodont and Selenodont types of molars. The Perissodactyla, as defined 



by the feet, axis, palate, etc., present us with the Symhorodont, Palmotheriodont, 



Tapirodont, and Selenodont types of dentition in the superior series, and the Hip- 



podont, Hyracodont, Palaotheriodont, and Trichechodont types in the mandibular 



teeth. The pentadactyle, plantigrade type, for which I have used the name Pro- 



hoscidia, presents us with the Trichechodont, Bathmodont, and Loxolophodont types 



of molar structure. Among Sirenians, 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 Trichecodont. Finally, the 



Marsupial group is unquestionably well defined, and here Phascolomys is Ptycho- 



dont; Hypsiprymnus Bunodont ; Petaums between Bun o d ont &nd Selenodont, and 



the Kangaroos and their gigantic extinct allies the Diprotodontidcs, Trichecodont. 



It is thus evident that the molar types are everywhere subordinated 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 construc- 

 tion of the feet. As to the modifications presenled by the canine and incisor teeth; 

 these exist within a still more narrow range of variation ; for instance, in the allied 

 genera Equus, Rhinaster, and Symborodon ; Sus and Phacochoenis ; Bathmodon and 



Eohasileus, 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.t 



* It was previously published in a separate form in "On the Primitive Types of Mammalia Educabilia," May 

 6 1873. An error occurs in this edition in the reversal by a lapsus calami of the positions of the types Omni- 

 vora and Anoplothermm. It is also important to note/that in the " Keport G. S.," p. 645, where it is stated 

 that " during the Eocene they (the orders) were in process of differentiation," etc., Maramalia Educabilia and 

 not Lissencephala, are referred to. 



t In the case of the Marsupialia the relation of the dental and extremital types may be reversed. Thus 

 we have pentadactyle plantigrade forms (Opossums) and (nearly) didactyle digitigrade forms [Macropus) in the 

 same order. Also Halmaturus and Diprotodon, both Trichecodonts, differ in the type of feet, as do the car- 

 nivorous Didelphys and Thylacinus, both Banodonts. 



