No. i.] OSTEOLOGY OF PCEBROTHERIUM. 1 3 



third one is very different ; it consists of a posterior pair of cres- 

 cents, internal and external, while the anterior portion of the 

 tooth is a long cutting blade with hardly a rudiment of the 

 antero-internal lobe. In the structure of D. 3 Pcebrotherium 

 thus agrees with the tragulines and the more ancient seleno- 

 donts generally ; seen from the outer side there are three cusps, 

 but only one internal one, the posterior. In Pcebrotherium, how- 

 ever, the anterior blade is very much elongated. Riitimeyer has 

 laid great stress upon the construction of this tooth in his clas- 

 sification of the selenodonts, but, as will be seen later, it is of 

 very doubtful value. D. 4 is composed of four crescents like a 

 true molar, but is proportionately broader. In the lower jaw 

 the three anterior milk-molars are very similar to their succes- 

 sors in the permanent series ; as to the first one, it is not certain 

 whether it is changed, though it probably is, at least in some 

 cases. Thus in all the adults which I have seen pm. 1 is im- 

 planted by a single fang, while in some young animals the first 

 milk-molar is inserted by two fangs, in others by only one. 

 D. 4 is like that of the selenodonts generally, consisting of three 

 pairs of crescents, of which the posterior pair is markedly 

 broader than the others. 



The milk dentition remains long in place, until the true 

 molars have all appeared and are in full use. 



II. The Skull (PI. I, Figs. 1-3, 7). 



In general appearance the skull of Pcebrotherium is decidedly 

 like that of the llama, though with many important differences. 

 The cranium is short and rounded, the face very long and taper- 

 ing ; the upper contour of the skull is nearly straight, the orbits 

 very large and prominent and situated further back than in 

 either of the living genera ; the occiput is rather high and nar- 

 row, the face very shallow, especially anteriorly, and the narial 

 opening is low, narrow, and nearly vertical. The zygomatic 

 arches are rather short and very slender. The cranio-facial axis 

 is straight, and the sagittal crest is long, but low and thin. 



Examined more in detail, the skull of Pcebrotherium exhibits 

 its tylopodan affinities very clearly, though, as would naturally 

 be expected, in a less extreme degree than in the existing 

 forms. The basi-occipital differs in shape in the two species ; 



