lO PATAGONIAN EXPEDITIONS I PALEONTOLOGY. 



able skulls and probably, as in the Macrauchenidse, was very small and 

 did not form a bulla. The nasals are usually of normal length, articulat- 

 ing with the premaxillse, but may be somewhat shortened, as in Thoathe- 

 ruim, or considerably so, as in DiadiapJionts. However, this reduction is 

 never comparable to that in the Macrauchenidse. 



The neck is always remarkably short in comparison with the length of 

 the limbs, and the cervical vertebrae have the vertebrarterial canal in the 

 normal position, perforating the transverse processes. The odontoid pro- 

 cess of the axis is always peg-like. The number of trunk-vertebrae is not 

 definitely known, for no complete back-bone has yet been found, but a 

 careful study of the available material renders it probable, that this number 

 did not exceed 21-22, 14-15 thoracics and 7-8 lumbars. The posterior 

 thoracic and lumbar vertebrae have semicylindrical, interlocking zygapophy- 

 ses. The sacrum is long and tapers so much posteriorly, that the shortness 

 of the tail is made quite certain, even in the absence of any known caudal 

 vertebrae. The ribs are of the ordinary ungulate type, and the sternum 

 has not yet been recovered. 



The limbs are long and slender, but the feet, except in Thoatherium, 

 are of only moderate elongation and the metapodials are usually short. 

 The scapula is large and has a prominent metacromion and a small acro- 

 mion, the two processes being quite near together, much nearer than in the 

 Macrauchenidae. The humerus is short and the fore-arm bones are always 

 separate ; the ulna is surprisingly little reduced, in view of the great degree 

 of digital reduction attained in the family. The elongation of the fore- 

 arm is not so great and that of the manus greater than in the Macrau- 

 chenidae. 



The pelvis has a decidedly equine appearance, though the division 

 of the iliac plate into dorsal and ventral processes is less marked, and the 

 tuberosity of the ischium is small in all of the genera in which the pelvis 

 is known. The limb-segments have somewhat different proportions from 

 those of the Macrauchenidae, the femur being relatively less, and the tibia 

 and pes more elongate. The femur has a strikingly equine character, 

 which is due to the proportions of the head and shaft, the high, bluntly 

 pointed great trochanter and the prominent third trochanter, which is larger 

 than in the Macrauchenidae. The tibia and fibula are always separate, 

 though the latter is slender and much more reduced than the ulna, and the 

 articulation with the calcaneum is retained in all the genera. 



