18%.] lv)l [Farr. 



from a very short-crowned tooth with distinct roots, to one with very 

 long crown in w^hich roots are not formed till animal becomes adult. 

 3. The gradual lengthening of the limb bones with the suppression of 

 the lateral digits and the concentration of the growth force in metapo- 

 dial iii, producing ultimately a monodactyl foot from a pentadactyl 

 ancestor. 



3. The continued reduction of ulna and fibula and their ultimate 

 coalescence witli the radius and tibia. 



4. Gradual increase in size from an animal not larger than a fox up to 

 the modern horse. 



Mesohijypus hairdi is an interesting intermediate stage in the evolution 

 of the horse ; though primitive in many respects, it had already made 

 considerable advance over its Uinta predecessor. 



The restoration here given is made from a nearly perfect skeleton 

 which enables us to make some improvements on the one already given,* 

 which, however, was as good as could be made with the material then 

 available. 



The lumbar vertebrae, sacrum, pelvis and a few of the posterior dor- 

 sals are from another individual reduced to proportion. Part of the 

 skull is also restored from another specimen. 



Mesoliippus occupies a position about midway in the line of descent of 

 the horse series. It presents the following advances over its Bridger 

 predecessor, Pachynolophus. 



1. The teeth are longer (vertically) and more complex, the interme- 

 diate cusps are better developed, and the transverse ridges are likewise 

 better developed and more nearly confluent with outer wall of tooth. 



2. The lateral metapodials are more reduced comparatively, and meta- 

 podial iii is much larger. In the Bridger form the phalanges of the 

 fifth digit are present, but M. bairdi has lost these. 



3. Both the ulna and fibula are more reduced than in the earlier form 



4. In M. bairdi, Pms. 2-4 are molariform, while in Pachynolophus 

 Pm. 4 only is molariform and is smaller than true molars. Epihippus, 

 the Uinta representative of the series, has Pms. 3 and 4 molariform, and 

 this is the only generic distinction between the Bridger and Uinta 

 genera. 



The orbit is commencing to retreat, though it is still over the molars, 

 the anterior border being directly over the posterior half of M. 1. In the 

 horse it is situated posterior to molar series, and we can trace a gradual 

 transition in the position of orbit up through the different genera from 

 Mesohippus to Equus. This shifting backward of the orbit brings 

 about a gradual elongation of the facial region of the skull. The alve- 

 olar border of the maxillaries is low, this of course being associated with 

 low-crowned, short-rooted teeth. 



From the character of the teeth we may judge of the life habits of 

 the animal. The teeth of the modern horse have very long crowns 



*Journ. o/Morph., Vol. v, No. 3. p. 337. 



