Volume V. December, i8gi. Number 3. 



JOURNAL 



MORPHOLOGY. 



ON THE OSTEOLOGY OF ME SO HIP PUS AND 

 LEPTOMERYX, WITH OBSERVATIONS ON 

 THE MODES AND FACTORS OF EVO- 

 LUTION IN THE MAMMALIA ■ 



W. B. SCOTT, 

 College of New Jersey, Princeton. 



The genus Mesohippus stands about midway in the line of 

 equine descent, .so far as that line is known, and is itself, very 

 probably, an actual ancestor of the modern horses, or at all events 

 is so closely allied to such ancestor as to answer all the pur- 

 poses of this study. Although the horse series is perhaps the 

 most complete of any that have yet been made out among 

 mammalian phylogenies, yet it is not my design to attempt a 

 description of all the members of this series, as that would 

 require a very extended and voluminous account, and is, besides, 

 quits unnecessary for the main object in view. This object is 

 to ascertain, so far as is yet possible, the mode or law of evo- 

 lution in a mammalian phylum, and as the place of MesoJiippus 

 in the equine phylum seems to be fairly well established, a com- 

 parison with existing forms will yield instructive results. 



The very number of well-preserved fossil equine genera is a 

 source of embarrassment, and renders the systematic position 

 of some of these forms a matter of grave doubt. Thus the 

 relation of the European genera AncJiilopJius, Anchitherium, and 

 Hipparion to their American contemporaries and to existing 

 species, is very far from clear, and to assume, as has been done, 



