1857.] OWEN PLIOLOPHUS VULPICEPS. 6? 



degree of difference hi their dental characters with that which has 

 influenced MM. Pomel and Gervais in subgenerically separating 

 Pachynolophus from Lophiodon ; and by the fact of the mandibular 

 teeth of the Hyracotherium leporinum being yet unknown. 



Lophiodon, Pachynolophus, Pliolophus, and Hyracotherium seem 

 thus to form so many subgeneric modifications of the same natural 

 family of Perissodactyle Ungulates ; and in the modifications of their 

 dentition, especially in the comparative simplicity of their premolars 

 as compared with those of the subsequently introduced Palceotheria, 

 and in the progressive approach to the molar type of the Chcero- 

 potamoids made by Pliolophus and Hyracotherium, they exemplify 

 the tendency to a closer adherence to the general ungulate type. The 

 third trochanter on the femur of the Pliolophus, PI. IV. fig. 5, t, 

 and the association of three metatarsals, in one portion of the matrix, 

 fig. 14, which appear to belong to the same hind-foot, confirm, how- 

 ever, the essentially perissodactyle affinities of that genus, and, there- 

 fore, of its close ally the Hyracotherium. 



In stating that these modified Lophiodonts are the most artio- 

 dactyloid of the Perissodactyles, no particular hypothesis is advo- 

 cated : there can be but one inference from this and the numerous 

 analogous facts that have already been made known. So, likewise, 

 in regard to the typical character of dentition, as manifested by 

 the number and kind of teeth, we find in this last eocene mammal 

 which has come to light a repetition of that remarkable adherence to 

 a more general mammalian character. The older Oolitic Mammals 

 exemplify a tendency to a type of dentition of a still higher gene- 

 rality than the Mammalian class, as, for example : — 



Mammalia of which the dentition resembles the general vertebrate 

 type by the back teeth exceeding 7 in number : — 



Genera. Formations. 



Thylacotherium Lower Oolite. 



Spalacotherium Upper Oolite "I v , , 



Triconodon Upper Oolite J ™ beck - 



Mammalia resembling the Mammalian diphyodont type in the 

 dental formula of 



• 3—3 1 — 1 4—4 3—3 3 — 3 i 4—4 A , 



t 3"=3> C i=i> V 4— 4> m 3—3 > ° X P —3 &Ild ™ 4=4 = 44 « 



Genera. Formations. 



Palceocyon Sables de Bracheux (or somewhat older). 



Coryphodon Plastic clay. 



Pachynolophus Calcaire grossier moyen. 



Lophiotherium Marnes lacustres d'Alais (Gard). 



Pliolophus London clay. 



Hyracotherium London clay. 



Palceotherium Paris gyps. 



Anoplotherium. . Paris gyps. 



Anchitherium Lignites de la Debruge, pres Apt. 



Dichobune Binstead. 



f2 



