EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHEEES 



621 



and fibula, the right cuboid and cuneiforms, most of 

 the unguals, and parts of many ribs and vertebrae) 

 were restored in plaster. (See fig. 536.) 



The formula for the vertebral column probably but 

 not certainly is cervicals 7, dorsolumbars 21?, sacrals 4. 

 A careful study indicates that there were 17 dorsals 

 and at least 3 lumbars. The presacral region is rather 

 short, the neck decidedly so, the back moderately so. 

 In adaptation the short neck, high anterior dorsal 

 spines, rather slender mid-dorsal spines, elevated but 

 not very broad lumbar spines, and long ribs indicate 

 a very deep-chested animal of about the same pro- 

 portions as Tapirus indicus, but considerably larger 

 (perhaps one-fifth) and slower in its movements, with 

 decidedly less development of the extensor muscles 

 of the back, as indicated by narrower dorsal spines 

 throughout. 



lamina of neural arch, thin, elevated neural spine re- 

 stored at the summit. Transverse measurement of 

 neural spine near the base 11 millimeters, anteropos- 

 terior measurement 23. 



All the upper portions of the neural spines of the 

 anterior dorsals are restored except that of D. 4, 

 which is complete. The base of the spine of D. 1 is 

 laterally compressed and much more extended ante- 

 roposteriorly than that of C. 7. Measurements, 29 

 millimeters anteroposterior, 21 transverse (at base). 

 In D. 2, D. 3 the spines are very stout and transversely 

 extended at the base. In the succeeding dorsals the 

 neural spines become gradually less expanded at the 

 base as they recede in height; but the basal trans- 

 verse width (35 mm.), the deep posterior concavity, 

 and the moderately limited anteroposterior diam- 

 eter (24 mm.) are decidedly the characteristic 



Figure 536. — Composite mounted skeleton of Palaeosyops leidyi 



Including skull, neck, and parts of the fore limb of the type of P. le 

 eitlier to P. leidrji or to the closely related P. roiustus. 



The atlas and axis belonging with skull No. 1544 

 exhibit the following characters: Atlas with large ver- 

 tebrarterial canal traversing inferior portion only of 

 transverse process (47 mm. broad), which is indented 

 but not perforated to the front as in LimnoJiyops 

 laticeps; first cervical nerve issuing from anterior por- 

 tion of the neural arch; broadly rugose keel on lower 

 posterior portion of the centrum; transverse measure- 

 ment of anterior cotyli 102 millimeters, vertical meas- 

 urement 40, transverse atlas (estimated) 223. Axis 

 also with broadly rugose inferior keel, neural spine 

 moderately extended anteroposteriorly, abutting an- 

 teriorly against upper portion of neural arch of atlas. 

 Cervicals 3-6 with neural spines and pleurapophyses 

 (cervical ribs) partly restored; pre- and postzyga- 

 pophyses placed in decidedly oblique planes facing 

 inward and outward respectively. C. 7 with narrow 



dyi (Am. Mus. 1544) and material from other individuals referred 

 Upper Bridger C and D. One-fifteenth natural size. 



features of all the dorsal spines as far back as 

 D. 12 inclusive; in D. 13 the neural spine begins to be 

 more laterally compressed or flattened and of some- 

 what greater anteroposterior extent. In D. 4 and D. 10 

 we have a typical spine complete to the tip. These 

 proportions of the neural spines somewhat resemble 

 those of Amynodon and are quite distinct from those 

 in the Oligocene titanotheres Megacerops and Meno- 

 dus, as well as from those observed in the horse, 

 tapir, and rhinoceros, in all of which the neural spines 

 are more extended anteroposteriorly than transversely. 

 The anterior dorsal spines are also higher and less 

 elongate anteroposteriorly than in Dolichorhinus. 



The planes of the zygapophyses are significant as 

 follows: Decidedly oblique planes of the prezyga- 

 pophyses and postzygapophyses characterize D. 1 and 

 D. 2, both vertebrae belonging with the skull (No. 1544) 



