4 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



on the other hand, the neck is greatly elongated and, in most of the 

 cervicals, the vertebrarterial canal passes through the neural arch, very- 

 much as in the Tylopoda, to which Macrmichenia was originally referred. 

 In both groups, the odontoid process of the axis retains its primitive 

 cylindrical form. The number of trunk-vertebrae is not positively known 

 for any genus of either family, but it is very probable that this number is 

 19 in the Macrauchenidae and 21-22 in the Proterotheriidae. The lumbar 

 and posterior thoracic vertebrae have zygapophyses of the semicylindrical, 

 interlocking type, which is especially characteristic of the Artiodactyla, 

 but is found also in the horses. The last two lumbars and the first sacral 

 have additional articulations by means of large facets on the transverse 

 processes. The sacrum is long, tapering much posteriorly, which indi- 

 cates that the tail was short, though no caudal vertebrae have yet been 

 found. 



The limbs are long and slender, but the feet are only moderately 

 elongate, except in one monodactyl genus ( Thoatherhmi) of the Protero- 

 theriidae. The scapula has both acromion and metacromion, which are 

 most prominently developed in the Macrauchenidae. No clavicle is known. 

 The humerus is short and has a very simple, pulley-like trochlea. The 

 fore-arm bones are separate in all of the Santa Cruz Litopterna and the 

 ulna is remarkably large, while the radius is relatively slender. The 

 femur has a prominent third trochanter and a large pit for the plantaris 

 muscle. The leg-bones are not ankylosed in any of the Santa Cruz 

 genera, but the fibula, though complete, is much more reduced than the 

 ulna and always retains an articulation with the calcaneum. 



They^g/are quite different in the two families, though both are char- 

 acterized by mesaxonic, or perissodactyl symmetry. In the Protero- 

 theriidae they have a very horse-like appearance, the median digit being 

 much enlarged and supporting the whole weight, while the lateral digits 

 are reduced to mere dew-claws. In one genus of this family, Thoatheriwn, 

 complete monadactylism is attained and only minute vestiges of the lateral 

 metapodials remain. The ungual phalanges have a decidedly equine 

 character. On the other hand, in the Macrauchenidae, the feet are func- 

 tionally tridactyl and almost isodactyl, the three digits being of approxi- 

 mately equal length and thickness, though the median, or third, is slightly 

 the longest. The ungual phalanges are short and heavy, almost nodular, 

 and have a rhinocerotic rather than an equine appearance. In both families. 



