EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



619 



and fifth lumbars are missing. They exhibit increas- 

 ingly broad centra and moderately broad transverse 

 processes. The zygapophyses are vertically placed, 

 and unlike those of Palaeosyops are slightly if at all 

 revolute. 



Sacrals. — The most distinctive character of the 

 sacrals is that the sacrum includes five vertebrae, the 

 fifth (S. 1) being due to the coalescence of an anterior 

 caudal. They measure 171 millimeters anteropos- 

 teriorly. The first and second sacrals and the anterior 

 portion of a third sacral enter into union with the ilium. 



Pelvis. — The innominate bones are preserved almost 

 entire (fig. 532). The left os innominatum measures 

 406 millimeters anteroposteriorly. The crest of the 

 ilium measures 203 millimeters transversely; the 

 superior border is thin and indented in the median 

 portion or crista iliaca. The rugose tuber sacrale 

 is narrower (80 mm.) than the rugose tuber coxae 

 (140 mm.). The elongate proportions of the innomi- 

 nate bones are partly due to the juvenile nature of 

 this individual. 



Fore limb oj type. — The proximal and distal portions 

 of the right humerus are preserved. The head and 



Figure 532. — Right os innominatum of Limno- 



hyops laticeps 

 Yale Mus. 11000 (type); Bridger C or D. One-.sixth natural size. 



greater tuberosity measure 122 milHmeters (antero- 

 posterior); the head measures 65 (transverse); there 

 is a wide and deep bicipital groove ; the proportions are 

 somewhat altered by crushing; the distal end of the 

 humerus measures 68 (transverse) ; the total width of 

 the distal articular surface is 55. 



The radius and ulna are complete. The radius is 

 distinguished posteriorly by a characteristic median 

 groove; it is 230 millimeters long; its humeral facets 

 measure 53 millimeters transversely and 27 antero- 

 posteriorly. 



The ulna is 307 millimeters in length; its distinctive 

 character (see fig. 511) is the downward extension on 

 the outer side of the olecranon of the rugose crest for 

 the triceps muscle. 



Palaeosyops 



The Eocene titanothere skeleton is best known in 

 members of the genus Palaeosyops, which was the 

 largest and the most heavily built titanothere of that 

 time, adapted to slow locomotion on soft ground along 

 water borders and in marshes and swamps. We have 

 the nearly complete skeleton of P. leidyi from the 



upper Bridger and parts of the more progressive P. 

 rohustus and of the ancestral P. major. A titanothere 

 family likeness is seen throughout the axial and 

 appendicular parts, but generic and subfamily differ- 

 ences, closely shared with Limnohyops, are apparent 

 throughout. 



Up to the end of the lower Bridger deposition we 

 may safely select the largest and most massive titano- 





FiGUKE 533. — Pelvis of Palaeosyops major 



Am. Mus. 13116; Bridger B 3. A, Ventral aspect: B, dorso-superior 



aspect. One-sixth natural size. 



there bones as belonging to Palaeosyops. The gravi- 

 portal tendency is especially manifest in the hind 

 limb, with its elongate femur and abbreviate tibia 

 and slight angulation at the knee joint. The broad 

 and spreading foot bones of both manus and pes 

 are especially distinctive. The manus is functionally 

 tridactyl rather than tetradactyl, owing to the reduced 

 condition of the fifth digit, which is set apart from the 

 other digits in a manner peculiar to this genus. The 



