1896.] 



157 



[Farr, 



low, while anteriorly it is much narrower and higher. Both superior and 

 inferior surfaces are plane and the sides are very strongly concave. The 

 third segment is evidently the first division of the mesosternum, and is 

 high and long and almost trihedral in cross-section. These separate 

 segments of the sternum are not coossified, and the surfaces for the articu- 

 lation of the sternal cartilages of ribs are not well shown. From the por- 

 tions of sternum described above we are safe in assuming that there were 

 at least six segments in the sternum of M. bairdi. 



The Scapula. 



The nearly complete skeleton from which the restoration given here- 

 with is made fortunately has the scapula very well preserved, and this 

 reveals quite an unexpected character, viz., the presence of a distinct 

 acromion. The only other Perissodactyl known to have retained this 

 process is Pachynolophus (Orohippus) of the Bridger Eocene. Marsh * 

 has described it in this genus as follows : "The scapula has a prominent 

 acromial process, which is com- 

 pressed and decurved as in some Fig. 3. 

 Carnivora." Mesohippus is the 

 only Perissodactyl known to have 

 retained this process until Oligo- 

 cene times, and it has thus been 

 retained longer by the horses than 

 by any other family of this order. 

 It is possible tliat future discoveries 

 may also reveal the presence of a 

 clavicle in Mesohippus, as it has 

 been discovered in the contem- 

 porary Oreodon eulbertsoni,\ and in 

 the latter genus it persists until 

 Deep River times, where it has 

 been found by Prof. Scott J in the 

 form which he has called Mesoreo- 

 don. The possession by both 

 Mesohippus and Pachynolophus of 

 this process would seem to justify 

 us in regarding the latter as the 

 Bridger ancestor of the horse line 

 of which Mesohippus is the White 

 River representative. The scapula 

 is wider in proportion to its height 

 than that of Equus. The anterior 

 margin is very thin and strongly 



*Amer. Jour. Sc. and Arts, Series 3, Vol via, 1874, p. 247 



+ A specimen in tlie museum of the University of Chicago reveals the presence of the 

 clavicle. 

 I Trans. Amer. Philos. Soc, Vol. xvii, p. 136. 



Scapula of M. bairdi, %. 

 a, from outride. 

 6, from behind. 

 c, from below. 



