MECHANICS OF LOCOMOTION 



749 



(1839.1, pis., vol. 4, Palaeotherium, pis. iii and iv), 

 are too incomplete to afford allometric ratios, but they 

 conform strictly to the universal principles of speed 

 and weight allometry. The elongate radius of Paloplo- 

 therium minus parallels that of the lightest cursorial 

 types of Equidae. In Palaeotherium medium the 

 radius exceeds the humerus in length, a subcursorial 



parallels that of the mediportal rhinoceroses like 

 R. (Dicerorhinus) sumatrensis; the elongate scapula 

 of P. magnum parallels that of the graviportal (black 

 rhinoceros) type, namely, R. (Opsiceros) hicornis. 



In the hind limb, similarly, the tibia of Palaeo- 

 therium medium apparently exceeds in length the 

 femur and presents a "subcursorial" ratio. In the 



FiGUEE 678.- 



C D 



-Limb structure of perissodactyls: Hind limbs of paleotheres 



Limb segments showing contrasts. A, Palaeotherim 

 (subgraviportal). 



medium (subcursorial); B, P. latum; and C, P. crassum (mediportal); D, P. magnum 

 After De Blainville, 1839.1. One-fourth natural size. 



ratio. Weight-bearing progression from P. medium 

 to P. crassum and P. latum is indicated in the broad- 

 ening of the collum ■ scapulae and shortening of the 

 radius. 



The fan-shaped scapula of Palaeotherium medium 

 parallels that of the earlier cursorial horses like 

 Mesohippus; the ovate-shaped scapula of P. crassum 



femur of P. latum, P. crassum, and P. magnum the 

 third trochanter is very low down on the shaft, as in 

 graviportal forms; this is paralleled in the modern 

 graviportal rhinoceroses such as R. (Dicerorhinus) 

 sumatrensis. The knee is strongly flexed, the patellar 

 facet being very oblique with reference to the shaft 

 of the femur, as in early amynodonts and rhinoceroses. 



