MECHANICS OF LOCOMOTION 



747 



This table appears to show that there is no constant 

 significance in the ratios of length between the scapula 

 and the humerus. In the Equidae, for example, the 

 scapula appears to be elongating with speed as we pass 

 from Mesohippus to Neohipparion, but the scapula of 

 the slow-moving Eippidion exceeds in length that of 

 the swift Neohipparion, and both are surpassed by that 

 of Toxodon. Similarly the abbreviation of the ilium 

 as compared with the elongation of the femur reveals 

 no contrast of speed or weight ratios. 



The indices, or relations of breadth to length, of the 

 ilium and scapula respectively afford the really sig- 

 nificant and important figures. 



SECTION 2. SYSTEMATIC COMPARISON OF THE 

 PECTORAL AND PELVIC ARCHES AND OF THE 

 LIMB BONES IN EIGHT FAMILIES OF PERISSO- 

 DACTYLS 



TAPIEIDAE AND LOPHIODONTIDAE 



THE TAPIRS 



In limb proportions the modern tapirs (Tapirus) 

 are typical persistent mediportal types. Even at 

 the present time they are in an earlier stage of 

 evolution than the light-limbed Eocene titanotheres 

 {Mesatirhinus) ; and they are also capable of more 

 rapid motion, as indicated by the greater length of 

 the metapodials. 



The scapula, with its distinctively tapiroid notch 

 in the anterior border, is intermediate in contour 

 between the cursorial and graviportal digitigrade 

 types. 



The fore-limb mediportal characters of Tapirus 

 are (a) rounded rather than angulate superior 

 border of scapula; (6) prominent tuber spinae of 

 the scapula; (c) no acromion scapulae; {d) promi- 

 nent great tuberosities of the humerus. A primi- 

 tive mediportal character of the humerus is the 

 equal development of the muscular attachments 

 on the inner and outer sides of the distal ends of 

 the humerus, as reflected in entocondyle and 

 ectocondyle of equal size. A primitive character 

 of both Tapirus and Heptodon is the asymmetry 

 of the distal rotula and capitellum of the humerus. 



Throughout all its parts the species T. terrestris 

 is slightly more cursorial in type than T. indicus. 

 This difference is observed in both the fore and hind 

 limbs; the olecranon of the ulna alone exhibits a 

 marked difference of character. 



The hind limb of Tapirus is of the primitive medi- 

 portal type. The femur is somewhat longer than the 

 tibia, the great trochanter is prominent, the second and 

 third trochanters are less elevated on the shaft than 

 in Heptodon. As in the fore limb, T. indicus is seen to 

 be more graviportal than T. terrestris, especially in the 

 weight of the shaft of the femur and prominence of 

 the great trochanter. 



THE LOPHIODONTS 



Heptodon is to be regarded as a precociously cursorial 

 lower Eocene relative of the heavy-bodied Lophio- 

 dontidae of Europe. Its radius equals the humerus in 

 length and the tibia equals the femur, whereas in 

 Tapirus the corresponding proportions are the reverse, 

 the humerus exceeding the radius, the femur exceeding 

 the tibia. This is the primitive mediportal condition. 

 The radius and ulna are relatively elongate, and the 

 olecranon of the ulna is erect. The tuberosity of 

 humerus is moderately developed. A primitive char- 



B C 



-Limb structure of perissodactyls: Hind limbs of 

 Heptodon and Tapirus 



A, Heptodon calckulus (family Lophiodontidae), one-fourth natural size; B, Tapirus 

 terrestris, and C, Tapirus indicus (family Tapiridae), one-sixth natural size. 

 Showing the contrast between a primitive cursorial tjiie (Heptodon) and a persistent 

 mediportal type ( Tapirus) . 



acter of Heptodon is the persistent acromial rugosity 

 of the spine of the scapula. 



The cursorial characters of the hind limb of Heptodon 

 are seen in the elongate tibia and the elevation of the 

 second and third trochanters on shaft. 



PALAEOTHEEIIDAE 



The adaptive radiation of the paleotheres (Palaeo- 

 tTierium, PaloplotJierium) into animals of greater or 

 less speed and weight was observed by Cuvier. The 

 bones of the fore limb, here figured after De Blainville 



