ORIGIN AND DEVELOPMENT OF THE EXISTING HORSES. 721 



and separated by a distinct trochlea or pulley-like depression from before back- 

 ward, and forming a perfect hinge joint in its articulation with the cuboid and 

 navicular bones respectively. The superior surface of this bone in both these 

 orders is strongly convex fore and aft, and is traversed by a deep groove passing 

 outward and backward which receives a corresponding elevation on the distal face 

 of the tibia. 



Another ungulate or hoof-bearing order to which they hold near relationship, 

 has been separated and defined by Prof. E. D. Cope in his Report on Captain 

 Wheeler's Survey West looth Mer., under the name Amblypoda, a term meaning 

 short-footed. They are as yet confined to the Eocene period exclusively, and are 

 found both in Europe and this country. In points of affinity to the hoofed orders 

 generally they occupy a most interesting and important position ; being in all 

 probability the oldest, and offering the most generalized condition known among 

 the ungulates. The brain capacity is exceedingly small in relation to the size of 

 the other parts of the skeleton, and from certain casts made from the brain case 

 itself we are warranted in assigning these animals a position among the low- 

 est mammalia; they are lower in brain development even than some of the Mar- 

 supials. The feet are very short, are provided with five fully developed toes, 

 and have their entire plantar and palmar surfaces applied to the ground, as in the 

 modern bears. The astragalus is greatly flattened from above downward, and is 



primitive and characteristic. It displays 

 upon its inferior surface flattened artic- 

 ular facets for both navicular and cuboid 

 bones which share the articulation about 

 equally. Upon the superior portion, the 

 surface articulating with the tibia, it is al- 

 most flat, a condition which must have 

 rendered the ankle joint capable of very 

 little movement and giving to these ani- 

 mals a peculiarly awkward and shambling 

 gait. It is not difficult to perceive that 

 these siTiall brained five-toed and planti- 

 grade Amhlypoda could easily have furn- 

 ished a starting point for both the Artio 

 and Perissodaciyla, and, as we have good 

 * I'f] \^)^'''KS reasons to believe, did give origin to 



r^/^ t^""^ the Proboscidea or elephants. Figs, i, 



2 and 3 illustrate the difference noticea- 

 ble in the feet of hind limbs of the three 

 Fig. I. orders. Fig. i represents foot of an 



Right posterior foot of a species of Coryphodon, Amblypod ; Fig. 2, that of a PeHssodaC- 

 after Cope; one-half natural size. ^y^^ ^nd Fig. 3 that of an Ariiodactyle. 



From the study of collections recently made by the writer in the lower Eocene 

 beds of Wyoming, Prof. Cope divides the Perissodaciyla into two sub-orders. 



