No. i.] OSTEOLOGY OF PCEBROTHERIUM. $7 



length of the posterior region of the cranium and the anterior 

 position of the paroccipital processes. 



In the lower jaw the horizontal ramus is relatively longer and 

 more slender than in the recent genera, the symphysis is much 

 shorter, and the ascending ramus much lower, in correspondence 

 with the more brachyodont dentition ; the angular process is very- 

 much longer, extending far back of the line of the condyle, but 

 much lower from above downwards. The condyle is more 

 extended transversely, and the articular surface for the post- 

 glenoid process much less developed, and in general the condyle 

 departs less decidedly from the ordinary ruminant pattern than 

 does that of the living Tylopoda. The coronoid process is 

 shorter, more regularly recurved, and more like that of the 

 Pecora : it is compressed and thin, and shows no' tendency to 

 assume the massive condition found in Camehis. There is a 

 rather small but deep masseteric fossa. 



Though the skull of the more ancient genera referred to the 

 cameline series is unknown, yet from the analogy of other ungu- 

 late lines we may infer with considerable confidence that several 

 features of the skull structure of Pcebrot/ierium indicate advance 

 and differentiation, as, for example, the posterior position of the 

 orbit, the shortened cranium, and very long face ; the great size 

 of the auditory bulla?, etc., etc. Indeed, in some respects Pce- 

 brotlierium is more modernized than the camel. 



The skull of Protolabis exhibits no very important changes ; 

 the nasals are somewhat shortened, and the anterior nares 

 higher and more obliquely placed than in Pcebrot/ierium ; the 

 face is deeper and the orbit rather smaller ; the face is con- 

 stricted in front of pm. 2, and the distance from the tip of the 

 premaxillaries to the infra-orbital foramen, which is still placed 

 above pm. 4, is considerably lessened ; the posterior nares are 

 situated further back, apparently altogether behind the molars. 

 The superior alveolus is continued some distance back of the 

 third molar. 



The skull of Procamelus (PI. I, Figs. 4-6) has two well-dis- 

 tinguished types of structure, one of which, P. occidcntalis, Leidy, 

 resembles the llama ; the other, P. angustidens, Cope, is more 

 like the camel (cf. Cope, No. 6, PI. LXXVII, Fig. 1, and our 

 PI. I, Fig. 6). In both the orbit is decidedly smaller than in 

 Pcebrotlierium and separated by a wider interval from the molars 



