No. i.] 



OSTEOLOGY OF PCEBROTHERIUM. 



49 



esting reminiscence of its previous condition, in that the deu- 

 teroconid is a conical tubercle, and that the anterior valley opens 

 inwards in front of it, as in the true molars of Lophiomeryx, 

 while in the molars of Agriochcerus the functionally equivalent 

 metaconid expands into a plate which closes the anterior valley. 

 Likewise, if we compare the fourth lower premolar of Thinohyus 

 from the John Day beds with that of Dicotyles, the same process 

 of development may be made out : first, the addition of the deu- 



7 1 ' 



me 



me 



Fig. B, p. 4 of Agriochce- 

 rus latifrons. pr, pro- 

 tocone. vie, deutero- 

 cone. op, tritocone. 

 ep, tetartocone. 



^m 



ec P r -£c 



en, 



me 



^m-en 



Fig. C, p. 4 of Oreodon 

 Culbertsoni. pr, pro- 

 toconid. vie, deutero- 

 conid. ec, hypoconid. 

 en, entoconid. 



FlG. D, p. 4 of Agriochce- 

 rus latifrons. (Let- 

 ters as in Fig. C.) 



teroconid, and then the elevation and division of the talon into 

 hypo- and entoconids. It is plain, therefore, that the parts of an 

 inferior molariform premolar are much more nearly homologous 

 with the corresponding elements of the molars than is the case 

 in the superior series ; the protoconid having the same position ; 

 viz., forming the antero-external crescent or cusp, as have also 

 the hypo- and entoconids. In the upper series only one element, 

 the protocone, is common to both premolars and molars ; and 

 while in the former it is the antero-external cusp, in the latter it 

 is the antero-internal. It is not asserted that the transformation 

 of the premolars always takes place in this manner, but it may 

 be regarded as the usual method. 



The camels of the John Day formation do not present any 

 important modifications of the dentition ; in some of them the 

 first upper premolar has but a single fang, and others are 

 decidedly reduced in size : the former Cope has erected into a 

 separate genus, Gomphotherium. It seems probable that these 

 forms are not in the direct line of camcline descent. 



In the succeeding formation, the Deep River {TicJiolcptus 

 beds of Cope), occurs the genus Protolabis, Cope, which is also 



