4c 4 ANATOMY OF VERTEBRATES. 



C. Bones of the Limbs. — These are cliiefly modified in their 

 proportions with reference to degree of swiftness of course in the 

 different species : which have diverged, in this respect, from the 

 okl tertiary type as exemplified by the Paloaothere, in two di- 

 rections, the extremes of which are now shown in the Rhinoceros, 

 fig. 165, and the Horse, fig. 300. The segments farthest from 

 the trunk are the seats of chief variety, and here the elongation 

 and attenuation of the bone is attended with suppression of certain 

 of the digits. 



The scapula is long in proportion to its breadth, and most so in 

 the Horse, fig. 300, 5i : the anterior angle is largely rounded off: 

 the spine developes no acromion, but gradually subsides as it ap- 

 proaches the neck of the scapula : it is situated nearer the hind 

 border in the Tapir (fig. 299, 5i), nearer the front border in the 

 Horse, mth concomitant differences in the areas of the supra- and 

 infra-spinal fossa? : in the Rhinoceros it equally bisects the blade- 

 bone, and is most prominent at its upj)er third. The coracoid is 

 a mere tuberosity in all. The front border or ' costa,' in the 

 Tapir, has a wide and deep notch. Macrauchenia differs most 

 from other Perissodactyles in the continuation of the spine, with- 

 out loss of height, to the neck of the scapula, above which it forms 

 a slightly produced angle and is perforated. 



In the Rhinoceros, fig. 165, the humerus is remarkable for the 

 strength of the tuberosities and deltoid ridge, and for the smooth 

 basal surfaces between the tuberosities and on the outside of the 

 external one. The medullary artery enters the back part of the 

 bone, and proceeds obliquely forward and downward. In the 

 radius, the surface for the ulna extends along the back part of 

 the rido-e boundino- that for the humerus. The two antibrachial 

 bones interlock at their distal ends by reciprocally adapted cavi- 

 ties and tuberosities. The usual eight bones are present in the 

 carpus : but the trapezium does not support a digit, and the unci- 

 forme is small and has only the digit answering to the fourth : 

 this, with the medius and index, being alone developed in the 

 Rhinoceros. The ilia are massive, short, and less expanded than 

 in the Elephant, sub vertical in position, concave anteriorly, and 

 also behind in the transverse direction. The terminal angle of 

 the rough thick crest is bifurcate. The ischia are relatively 

 longer than in the Elephant, with thick outwardly-bent tuberosi- 

 ties. The ischio-pubic symphysis is prominent. The lumbo- 

 iliac ano'le is 125°. 



The head of the femur is impressed by a deep semicircular pit 

 at its margin. The third trochanter, fig. 165, 65, is a remarkable 



