Io6 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



but quite prominent sagittal crest. The tympanic bone is very small, 

 scale-like and flattened, not forming a bulla, and with an irregular meatus, 

 which is not prolonged into a tube. The orbits are partially open behind 

 and the horizontal rami of the mandible are very slender. The especial 

 peculiarity of the skull is, however, to be found in the great reduction of 

 the nasals and elongation of the anterior nares, which indicate the 

 presence of at least an incipient proboscis. 



The neck is very long, in striking contrast to the short neck of the Pro- 

 terotheriidse, yet the odontoid process of the axis retains its primitive, 

 peg-like shape. In no other ungulate with a neck of comparable length, 

 does the odontoid fail to assume the concave, spout-shaped form. All 

 the cervical vertebrae, except the atlas, are greatly elongated and much 

 resemble those of the camel and, as in the latter, the vertebrarterial canal 

 traverses the neural arch, except in the first and sixth vertebrae, in which 

 it still perforates the transverse process. Thus the shifting of the position 

 of the arterial canal is not so complete as it afterwards became in 

 Macrauchenia. The number of trunk-vertebras, though not yet known 

 with complete certainty, should very probably be stated as nineteen, 13-14 

 thoracics and 5-6 lumbars, another difference from the Proterotheriidae, 

 in which the number can hardly have been less than twenty-one. As 

 in the latter family, the long, tapering sacrum is indicative of a very 

 short tail. The sternum is very peculiar ; the presternum and anterior 

 segments of the mesosternum are long, very narrow and deep dorso-ven- 

 trally, while the hinder segments of the mesosternum are broad and 

 depressed. 



The limbs are long and slender and the limb-segments are proportioned 

 differently from those of the Proterotheriidae ; the upper arm is short and 

 the fore-arm long, the thigh long and the leg short. The scapula is broad 

 and has a large acromion and very large metacromion, the latter about 

 midway in the course of the spine. The fore-arm bones are separate in 

 the Santa Cruz genus and the ulna is relatively heavy, the radius quite 

 slender. The long femur has a very large great trochanter and a well- 

 developed third trochanter, both of which are much more prominent in 

 the Santa Cruz than in the later representatives of the family. The leg- 

 bones, which have coalesced in Macrauchenia, are separate in Theosodon 

 and the fibula is quite slender. 



The feet are very different in appearance from those of the Protero- 



