ITS ANCESTORS AND RELATIONS. 29 



form a family Lophiodontidce. Some of these ani- 

 mals * present a very distinct advance in evolution 

 upon Phenacodus, an advance in some respects so 

 great as to move them, according to Cope, into a dis- 

 tinct ordinal division of the Mammalia. This con- 

 sists mainly in a modification of the form and re- 

 lations of the bones of the carpus and the tarsus 

 from their primitive condition, which modification 

 persists in all the more recent forms of true ungu- 

 lates, both Perissodactyle and Artiodactyle, and the 

 absence of which is the principal distinction between 

 them and the Proboscidea (elephants) and the Hyra r 

 coidea. 



The bones of the second row of the carpus no 

 longer stand exactly below the corresponding bones 

 of the first row, but are all shifted a little way to 

 the inner side of the foot, a change which is facili- 

 tated by the disappearance of the first digit, and 

 which, with certain alterations in the form of the 

 articular surfaces, tends to produce a more perfect 

 interlocking of these bones one with another, and 



* Madame Marie Pavlow has shown that under the name 

 of Hyracotherium some very different forms have been con- 

 founded, the type species of Owen being the most primitive, 

 and perhaps identical with Cope's PJienacodtis, while the Amer- 

 ican H. venticolum, of which the whole skeleton is known, and 

 to which the description in the text chiefly applies, is a much 

 more advanced form. 



