EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



623 



characteristic of all titanotheres, to a point within 

 115 millimeters of the glenoid border; at this point 

 the fossa measures 104 millimeters in front of the 

 spine; the postscapular fossa (origin of infraspinatus, 

 teres major, teres minor muscles), on the other hand, 

 is triangular, the broadest point, 131 millimeters, being 

 near the superior border; the posterior border rises 

 thus in a nearly straight line from the glenoid to the 

 superior border and then sweeps around in a uniformly 

 convex curve to the point above the anterior notch. 

 This scapula conformation is remotely paralleled by 

 that of Tapirus indicus, which, however, differs in the 

 greater depth of the supracoracoid notch; it also 

 resembles that of certain early rhinoceroses {Caenopus 

 occidentalis) , which, however, lacks the anteroposterior 

 breadth; it is also somewhat similar in type to that 

 of Rhinoceros sondaicus, differing again in the greater 

 breadth and the absence of retroversion of the spine. 

 These comparisons are naturally valuable not as in- 

 dicative of relationship but as pointing to analogous 

 development of the shoulder muscles and gaits of these 

 animals; they are distinctively noncursorial. The 

 breadth above the glenoid, including the coracoid 

 process, is 87 millimeters, the width of the neck is 80. 

 The spine rises gently from above the neck with a 

 re trover ted border reaching its widest point 210 milli- 

 meters above the glenoid and 144 millimeters below 

 the superior border; at this point the spine rises 50 

 millimeters above the postscapular fossa. 



The humerus of P. leidyi found with the skull (No. 

 1544) is intermediate in its robust proportions, which 

 are between those of Tapirus indicus and those of 

 Rhinoceros (Dicerorhinus) sumatrensis. In general 

 the muscular processes are about as strongly devel- 

 oped as in Tapirus indicus. The proportions of the 

 bones of the fore limb are accurately determined from 

 bones of both sides found with the skull. The upper 

 arm (humerus) (310 mm.) is considerably longer than 

 the forearm (ulnoradius) (235 mm.), as it usually is in 

 slow-moving, animals. The proportions of the humerus 

 are similar to those in Tapirus indicus, the approxi- 

 mate breadth across the great tuberosity being 103 

 millimeters and the total distal breadth 92; the lesser 

 tuberosity (insertion of subscapularis') and the greater 

 (insertion of supraspinatus and infraspinatus) are 

 moderately prominent; the greater tuberosity rises 

 into a platelike crest very characteristic of the titano- 

 theres; it sends in an internal hook; the bicipital groove 

 is thus broad and shallow; the highest point of the 

 deltoid crest (insertion of pectoralis major and del- 

 toid) is 119 millimeters below the head, it is slightly 

 retro verted; below this point it subsides gradually 

 into the shaft; the supinator ridge is sharply defined, 

 but not hooked, its summit being 109 millimeters above 

 the trochlea; the entepicondyle or internal eminence is 

 relatively low and obtuse; distally the radial trochlea 

 of the humerus is divided unequally into a smaller, 

 i01959— 29— VOL 1 i3 



less convex internal portion and a larger, more de- 

 .cidedly convex external portion (the tuberculum). 

 Earle (1892.7, p. 357) has directed attention to the 

 fact that this conformation of the trochleae differs 

 somewhat from that of other Perissodactyla; it is 

 correlated with the more elevated and plane internal 

 himaeral facet and the more depressed and concave 

 external humeral facet of the radius. 



The elevation of the inner side of the front face of 

 the radius is characteristic of titanotheres but is seen 

 also to a considerable extent in Amynodon. The width 

 of the humeral trochlea is 63 millimeters. There are 

 broad and deep anconal and supratrochlear fossae 

 but no perforation; the upper end of the radius 

 measures 63 to 67 millimeters transversely. 



The shaft of the radius arches well away from the 

 ulna and expands distally to a width of 62 to 67 milli- 

 meters. There is a distinct extensor groove in the 

 anterior distal face. 



The ulna has a stout olecranon and straight tri- 

 hedral shaft well separated from that of the radius, 

 with a broadly transverse section, of which one face 

 presents directly forward and is hollow or grooved 

 above and flattened below. 



The right carpus belongs to the same individual as 

 the skull (Am. Mus. 1544). The specific character of 

 the carpus is its great breadth (91 mm.) as compared 

 with its maximum depth (54 mm.) measured from the 

 lunar to the bottom of the unciform. Similar broad 

 and low proportions characterize each element. The 

 lunar is especially distinctive, measuring 36 milli- 

 meters transversely, 31 vertically, with an exception- 

 ally broad facet for the magnmn (fig. 537); a small 

 trapezium was present but is not preserved in this 

 specimen. In this, as in all other titanotheres, there 

 were large facets on the inner side of the magnum and 

 unciform for Mtc II and Mtc III, respectively. 



The metapodials (Am. Mus. 1544, 1550) are rela- 

 tively broad and distally expanded; the lengths or 

 vertical diameters are, Mtc II, 107 millimeters; 

 Mtc III, 112; Mtc IV, 97; Mtc V, 75. Mtc III has 

 a vertical diameter of 112 millimeters and a transverse 

 diameter distally of 46; the shaft of each has a rela- 

 tively broad and shallow flattened section, this being 

 a characteristic family feature. The metapodial dis- 

 placement is very marked, Mtc II and Mtc III 

 abutting widely against the magnum and unciform, 

 respectively. The proximal portion of the shaft of 

 Mtc II presents a small facet to which the trapezium 

 was attached. The three phalanges of D. Ill meas- 

 ure 39, 26, and 23 millimeters in length, respectively. 

 The distal phalanges are broadly rugose (tr. 35 

 mm.) with a deep medial cleft; the upturned distal 

 facets of the second phalanges indicate that when 

 the foot was drawn backward the unguals were more 

 sharply extended on the second phalanges. Even 

 with this reservation the fore foot was broad and 



