TYPES OF MOLAR TEETH OF MAMMALIA EDUCABILIA. 87 



Third, Of the relations of canines and incisors. 



With regard to the significance of the three types of feet, Proboscidian, Perisso- 

 dact^le, and Artiodactyle, it has been ah-eady remarked: "It is to be observed that 

 the lines of Ungulafa, Quadrumana, and Carnivora originate in plantigrade types, a 

 state of things quite predominant among the lower series or Lissencephala (smooth 

 brains). It is universal in Edentata and very usual in Rodentia and Insectivora. 

 The lower forms of Marsupialia and all of the Monotremes present it. In the Mar- 

 supials, 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 Batrachia 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 run- 

 ning or projectile force in leaping. 



The connecting points between the different types of foot-structure among the 

 Mammalia Educabilia are as obvious as in the case of the types of molar structure. 

 Examples may be adduced as follows : — 



Artiodactyla. 



Approximations to the Perissodactyla are to be seen in Hippopotamus in the 

 increase in development of the lateral or first and fourth digits, thus equalling 

 the number in the fore foot of Tapirus and Brontotherium, though preserving the 

 equality of the two median digits. But an inequality of these digits appears in 

 the genera Anoplotherium and Ccenotherium, as has been statedf in the following 

 language: ^"^ In Anoplotherium secundarium the digit ii is developed in each foot, 

 though not nearly so long as Hi, which is nearly symmetrical in itself. There is 

 an approach to the same structure in the manus of Ccenotherium" The only 

 approximation to the Proboscidian type is to be seen in the shortening of the 

 metapodial bones in Hippopotamus, a point of very inconsiderable value, 



Perissodactyla. 

 Approximation to the preceding order is made in the anterior foot of Bronto- 

 therium, in which, according to Marsh, there are four toes of nearly equal size.J 

 Approximation to the Proboscidia is seen in Symborodon, where the cuboid facet 

 of the astragalus is rather larger than in Rhinoceros, and developed much as in 



* Mammalia Educabilia, p. 8 ; Hayden's Geological Survey, 1873, p. 647. 

 t Huxley, Anatomy of the Vertebrated Animals, p. 321. 

 + American Journal of Science and Arts, 1873, p. 486. 



