647 



at the shoulder 1.09 meters (3 feet 6 inches). Alto- 

 gether the vertebral column is mechanically superior 

 in the strength of its muscular attachments to that of 

 Palaeosyops. We observe certain analogies to the 

 spinal column of Equus. The backbone is adapted to 

 the support of the long, depressed head; the broad 

 neural spines of the dorsal vertebrae serve for the 

 attachment of the ligaments and muscles supporting 

 the cranium. The spines in the lumbar region were 

 also deep, indicating the presence of powerful running 

 muscles. This apparent adaptation of the backbone 

 in the lumbar region to a strong running action is to 

 our surprise not correlated with length of limb or of 

 foot, because both the humerus and the hind feet are 

 relatively abbreviated. The median metatarsal meas- 

 ures only 120 millimeters. Other striking characters 

 of the restoration are the broad inferior lamellae of 



FiGUHE 581. — Atlas referred to Dolichorhinus sp. 



Am. Mus. 1844; Uinta C; dorsal view. Ttie arrows indicate the 

 course of the first spinal nerve and vertebral artery. One-third 

 natural size. 



the cervical vertebrae (C. 3-C. 

 ments in Am. Mus. 1843 are: 



6). The measure- 



Actual iength curve of back, axis to spine of posterior 



1. 35 

 Length of neck lacking axis — that is, C. 2-C. 7 .30 



Vertebral column. — The vertebral formula as pre- 

 served in Am. Mus. 1843 appears to be cervicals, 

 7; dorsals, 15-1-; lumbars, 4; sacrals, 3-4. The exact 

 number of dorsals is not revealed by this specimen. 

 In the specimen of Dolichorhinus longiceps described 

 by Eiggs (1912.1, p. 31), the formula is, dorsals, 17; 

 lumbars, 4; sacrals, 4. In Palaeosyops the number 

 is not certainly known. In two genera of Oligocene 

 titanotheres {Brontotherium, Menodus) the number 

 of dorsolumbars is believed to be 20. 



The atlas (fig. 581) is partly known from a specimen 

 in the American Museum (No. 13164) and fully known 

 by comparison of this specimen with a much larger 

 atlas (Am. Mus. 1844). It is moderately elongate; its 

 breadth is 203 millimeters. The vertebrarterial canal 

 traverses the base of the transverse processes. The 

 atlas is narrower transversely than that of Palaeosyops 

 and it has narrower pleurapophyses (201 mm. in 

 Metarhinus as compared with 240 in P. rohustus) ; the 

 superior border of the cotylus is deeply concave 

 anteriorly, the dorsal prominence (neural spine) is 

 larger, the cotyli are larger, and the articular sur- 

 faces for the axis form a more deeply concave sinus. 



