LOPHIODOXTID.^. 629 



the usual posterior keel. The corresponding extremities of the lateral 

 metatarsals are unsymmetrical, the adjacent part of the condyle being more 

 prominent, and the free portion shortened. In this respect they differ ma- 

 terially from those of Hyrachyus, where the lateral metatarsals are nearly 

 symmetrical, having the same character as the median one. 



The following comparisons maybe made between the limbs oiHyracothe- 

 rium and other Perissodactyla. The coracoid process is larger than in either 

 Triplopus or Hyrachyus. The proximal part of the humerus is a good deal 

 like that of both those genera. In the distal extremity the external condyle 

 is more prominent outwards than in either. The disproportion in diameters 

 of the ulna and radius is greater than in Hyrachyus, but not so great as in 

 Triplopus. In the carpus Hyracotherium is more like Hyrachyus than Tri- 

 plopus, in the great inequality between the anterior faces of the trapezoides 

 and magnum, from which it results that the metacarpus II has consider- 

 able contact with the latter. In Trtplopus there is very little such contact, 

 as the trapezoides presents as large an external face as the magnum. The 

 unciform is also wider in the two genera with four metacarpals. There is 

 considerable resemblance between the bones of the posterior limb of Hyra- 

 cotherium and Hyrachyus, but the great trochanter of the femur is larger in 

 the former. The proportions are more slender in the H. rentorum than in 

 the Hyrachyus- ayrestis, but it cannot be assumed that such a character will 

 distinguish all tlie species of both genera. 



Affinities. — Outside of the close relationships to other genera of Lophi- 

 odonticlce, there are resemblances to the Anchitheriidce. The anterior cingular 

 tubercle of the superior molar teeth of the family to which Hyracotherium 

 is here refen-ed, is less developed in this genus than in most of the othei-s, 

 while it is represented by a ledge in Anchiilierium. The humerus femur 

 and tibia, are much alike in the two genera; so also is the zygapophysial 

 articulation of the lumbar vertebrae. The elongate coracoid occurs in the 

 Anchitherium hairdi. The differences in the feet, the forearm, and the axis 

 are, however, great. The genus Triplopus presents an entirely intermediate 

 condition of the fore leg and foot (hind leg unknown), but the crests of the 

 molars are continuous and transverse, a structure which indicates for that 

 genus a position in the line of Hyracodon or Rhinocerus, rather than Anchi- 



