264 THE STRUCTUEAL EVIDENCE OF EVOLUTION. 



Quadrumana, and Carnivora originate in plantigrade types, a 

 state of things quite predominant among the lower series or Lis- 

 sencephala (smooth brains). It is universal in Edentata and very 

 usual in Rode7itia and Insectivora. The lower forms of Marsu- 

 pialia and all of the Monotremes present it. In the Marsupials, 

 Rodents, Ungulates, and Carnivores, we have series whose highest 

 expression is in the most highly digitigrade genera."* To this it 

 may be added that the lower terrestrial vertebrates are plantigrade, 

 with some exceptions. Thus in some Anurous BatracMa there is 

 a partial digitigradism ; the only digitigrade Reptilia are some 

 Dinosauria, especially such carnivorous forms as Lcelaps ; all 

 birds are digitigrade. The digitigrade modification evidently has 

 reference to speed in running, or projectile force in leaping. 



The connecting points between the different types of foot- 

 structure among the Ma7nmalia Educahilia are as obvious as in 

 the case of the types of molar structure. Examples may be ad- 

 duced as follows : 



Carnivora. — In all of the genera of the Eocene Carnivora which 

 I have had the opportunity of examining, excepting 3Iesonyx, 

 namely, Amhlyctonus, Oxycena, Prototomus, Didymictis, the tibio- 

 astragalar articulation is of a primitive character. The astragalus 

 is flat, and the applied surfaces are nearly a plane, and without 

 the pulley-shaped character seen in existing Carnivora ; as dogs, 

 cats, and in a less degree in the bears and in other Mammalia with 

 specialized extremities, as Perissodactyla, Artiodactyla, etc. The 

 simplicity of structure resembles, on the other hand, that found 

 in the opossum and various Insectivora, Rodentia, and Quadru- 

 mana, and in the Proioscidia, most of which have the generalized 

 type of feet. The structure indicates that the carnivorous genera 

 named were plantigrade — a conclusion which is in conformity 

 with the belief already expressed that the Mammalia of the Eocene 

 exhibit much less marked ordinal distinction than do those of the 

 Miocene or the recent periods. It is, indeed, questionable whether 

 some of the genera here included in the Carnivora are not gigantic 

 Insectivora, since the tibio-tarsal articulation in many, the separa- 

 tion of the scaphoid and lunar bones in Mesonyx, the form of the 

 molars, and the absence of incisor teeth in some, are all character- 

 istic of the latter rather than the former order. 



Artiodactyla. — Approximations to the Perissodactyla are to be 



*" Mammalia Educabilia," p. 8; Hayden's "Geological Survey," 18Y3, p. 647. 



