No. i.] OSTEOLOGY OF PCEBROTHERIUM. 29 



Naturally, the prezygapophyses are modified to fit them. In the 

 llama these processes appear irregularly, sometimes present on 

 one side and not on the other, and are often absent altogether. 

 In Pcebrotherium a slight beginning of the same structure may 

 be observed on some of the lumbar vertebrae (PI. II, Fig. 23). 

 In this genus, also, the postzygapophyses are more angulated 

 than in the llama, the superior and inferior articular surfaces 

 not passing into each other by such a gentle, regular curvature. 

 No sacral or caudal vertebrae are preserved in any of the spec- 

 imens at my disposal ; but if we may judge from the analogy of 

 its contemporaries, Pcebrotherium probably had a longer tail than 

 the modern representatives of the group. 



V. The Ribs. 



The ribs of Pcebrotherium differ decidedly from those of the 

 camel and llama in their much greater slenderness. The ante- 

 rior ribs are short and flattened, but by no means so broad as in 

 the existing species ; while those of the median and posterior 

 regions are much more slender than is usual among recent arti- 

 odactyls, and are rather to be compared to the ribs of carni- 

 vores. This is, however, usually the case among the more ancient 

 ungulates, and Hyrax still retains this type of rib-structure. In 

 the existing Tylopoda, and especially the camel, the ribs are 

 very irregularly curved, but in Pcebrotherium they have only the 

 usual outward and forward curvature. 



VI. The Fore Limb. 



The scapula (PI. II, Figs. 24, 25) is, on the whole, rather more 

 like that of the llama than that of the camel, though individ- 

 ual variations of considerable extent occur, which approximate 

 the condition found in Camclus. In the latter the scapula is 

 much higher and narrower than in the llama ; the spine rises 

 nearer to the glenoid cavity, and runs in a nearly median posi- 

 tion, dividing the blade into nearly equal pre- and post-scapular 

 fossae ; the acromion is less developed, the neck is longer and 

 broader, the coraco-scapular notch higher and more gently curved, 

 and the coracoid much more massive and rugose. In Pcebro- 

 therium the neck is very slender and contracted (in some speci- 



