No. i.] OSTEOLOGY OF PCEBROTHERIUM. 53 



cents arranged in transverse pairs, of which the anterior pair is 

 by far the largest. In Procamelus, to which genus I attribute the 

 "milk dentition of a camel " figured by Cope (No. 6, PI. LXXVII, 

 Fig. 4), the character of the temporary molars has undergone an 

 important change. D. 2 is like pm. 2, consisting of an elongated 

 and trenchant external cusp, and an imperfect internal crescent 

 formed by the elevation and extension of the anterior and pos- 

 terior basal cusps in the manner already described. D. 3 is the 

 longest tooth, antero-posteriorly, in the temporary series. It 

 consists of two pairs of crescents, of which the anterior pair is 

 the smaller and especially narrower: the rudimentary antero- 

 internal cusp of Pcebrotherium has now become a complete cres- 

 cent. The accessory anterior basal cusps of the White River 

 genus now forms a transverse "compression fold" across the 

 whole of the front face of the tooth. D. 4 is like a true molar 

 in constitution, and has a fairly well developed basal pillar at the 

 junction of the two inner crescents. The upper milk incisors 

 and canines, as well as the entire lower temporary dentition, are 

 unknown to me. 



In the recent genera of the Camelidce the milk molars are 

 very much reduced. In the upper jaw D. 3 has a compressed 

 anterior half, and is of irregular molar form. D. 2 is rudimen- 

 tary. In the lower jaw D. 4 is of the usual selenodont pattern ; 

 D. 3 is very much reduced with the anterior portion trenchant, 

 and D. 2 is wanting. There is thus in the cameline phylum a 

 similar change in the character of the milk dentition to that which 

 occurs in the true ruminants, Gelocus and Prodre7notJierium 

 agreeing in this respect with Pcebrotherium and the tragulines. 

 As I have pointed out elsewhere (No. 31, p. 370), the same 

 transition occurs in the Oreodoutidce, Oreodon having the older 

 type of milk dentition, in which D. 3 consists of a posterior pair 

 of crescents and an elongated antero-external blade, but no 

 antero-internal cusp, while in Merychyus this tooth is molariform 

 and consists of two pairs of crescents. The transformation has 

 occurred, therefore, at least three times independently among 

 the selenodonts, in the true ruminants, the camels, and the oreo- 

 donts. 



II. The Skull. 



The skull of the more ancient genera of the tylopodan series, 

 Pantolcstes, Homacodon, and Leptrotragulus, is quite unknown ; 



