140 Bulletin American Musemn of Natural History. [Vol. X, 



leading to the final loss of the first and second pair of superior 

 incisors. In Procauielus occidentalis these teeth have completely 

 disappeared in the adult, but vestiges of them have been found in 

 the young, a fact which was demonstrated by Cope. 



Pliaucheuia furnishes us with the next step in which the second 

 premolar has disaj^peared from the lower jaw, leaving the first, 

 third and fourth of this series. The superior premolar dentition 

 of this genus has not yet been found, but it is highly probable that 

 some of the species will show a loss of the corresponding tooth in 

 the upper jaw. It is highly probable that at this stage, or the one 

 preceding, three diverging lines took origin, of which one con- 

 tinued into Aiichcuia, another into Caiuclus, and a third into 

 Camelops and Eschatius. 



From Pliaucheuia the transition is easy to Camel us ^ in which 

 there are three premolars in the upper and two in the lower jaw. 

 In Aucheiiia the premolars are still further reduced, there being 

 two above and only one below. The first evidence of the peculiar 

 buttress found in the lower molars of this genus is seen in 

 Procamelus gracilis, and on this account I am of the opinion that 

 this species is the ancestor of the Llamas. It is highly probable 

 that the species migrated to South America at the close of the 

 Miocene, and that the intermediate links between it and the living 

 genus will be found in that country. Camelops could have easily 

 been derived from Pliaucheuia spatula, requiring the loss of only 

 two premolars in the lower jaw and probably one above ; in a like 

 manner Eschafius follows Camelops and carries dental reduction to 

 the extreme limit reached in the Camel group. 



In this attempt to indicate the more exact specific evolution of 

 the grou}) one is necessarily handicapped by want of knowledge 

 of the osteology of many species which are at present rejjresented 

 by fragmentary remains only. I doubt not that it will be materi- 

 ally altered when we come to have a more perfect understanding 

 of these forms. I give herewith a table of distribution in time, of 

 the Cameloids treated of in the foregoing pages. 



