1896.] ibo [Farr. 



developed, the bone is lower and broader, the neck is more constricted 

 proportionately than in the horse. In the latter the anterior border of 

 the scapula is not rounded as in Mesohippus. The ulna is very much 

 reduced in AT. bairdi, and the radius is enlarged to sustain the weight of 

 body. The ulna is distinct from the radius through the whole of its ex- 

 tent, the two bones not being coossified even in old individuals. Below 

 the proximal half the bone is much compressed and tapers rapidly to- 

 ward the distal end. This gives it a frail character so that it is almost 

 always broken away in fossilization, and only recently have specimens 

 been found which permit an accurate determination of its character. 

 The distal end is not compressed as it is higher up, but is round in cross- 

 section and bears a facet for the cuneiform. A rudiment of the fifth meta- 

 carpal persists. All the metacarpals and their phalanges are somewhat 

 shorter and less massive than the metatarsals and the phalanges of the 

 hind foot. The pelvis is thoroughly equine and yet differs in many 

 minor characters from that of the horse. It is narrower in proportion to 

 its length than that of the latter. The ilia expand less abruptly, the crest 

 is narrower and more elongate proportionately, and the ischia do not 

 bend upward posteriorly as in the horse, but are in a straight line 

 with the long axis of the ilia. The obturator foramen is more elongate 

 and narrower transversely, and the pelvic outlet is higher and narrower 

 proportionately than in the modern equine. 



The fibula was complete in M. bairdi ; was very much reduced in size 

 and was coossified with the tibia. The proximal end is quite small, the 

 shaft is filiform, while the distal end alone is quite large and forms the 

 external malleolus articulating with the astragalus, and in extreme ex- 

 tension of the foot also with the calcaneum. The fibula remains com- 

 plete until John Day times, for in Mesohippus (Anchitherium) prcestans 

 Cope from this formation it is retained in its entirety. 



The hind limbs are much longer than the fore limbs, more so propor- 

 tionately than in the horse, so that the rump must have been much ele- 

 vated above the withers if the different elements of the limb were not 

 very much more flexed on each other than would seem justifiable, judg- 

 ing from recent animals. Many of the White River animals had a curved 

 arched back instead of a straight back as in the horse, e. g., Hyoenodon, 

 Leptomeryx, etc. This is shown by the character of the centra of the ver- 

 tebrae. The great individual variations met with in M. bnirdi have been 

 noticed by every investigator who has studied a series of specimens of 

 this species. These variations are principally in the limbs and teeth. 

 Some of these have already been noted. In several individuals the 

 three cuneiforms of tarsus are all coossified into a single compound 

 cuneiform. Usually the ento- and meso-cuneiforms are united. 



There is usually a moderately large contact of metatarsal iii with 

 the cuboid, this latter usually extending below the level of the ecto- 

 cuneiform, so that all contact of metatarsal iii with cuboid is lateral. 

 In some specimens there is a slight extension outwardly of the proximal 



PROC. AMER. PHILOS. SOC. XXXV. 151. U. PRINTED SEPT. 5, 1896. 



