218 Annals of the Carnegie Museum. 



Miocene. It has been pointed out by Matthew and the writer that 

 in the Lower Arikaree deposits of the plains we not only have many 

 new genera, but those which are related to forms found in the John 

 Day beds are always farther along in the trend of their evolutionary 

 development, showing that they belong to a later period than the 

 John Day. The Upper Harrison beds should be regarded as sedi- 

 ments representing the transition period between the Lower and the 

 Middle Miocene. \Ye have indeed some reason for believing that the 

 Proboscidians were already in this country at the close of the Lower 

 Miocene, or at the very beginning of the Middle Miocene. ^^ 



Even granting that a long time elapsed between the formation of 

 the deposit in which Promerycochcerus occurs and the deposits in 

 which AIerycoch(erus is found, we are nevertheless confronted in the 

 anatomy of Promerycochcerus with features which are obstacles to 

 our regarding this genus as the direct ancestor of Merycochcerus. In 

 contending with these obstacles we might resort to an explanation 

 based upon atrophy and hypertrophy, but such an explanation gives 

 no idea of the reason why in one case an organ should have been 

 diminished and in the other functionally increased in power, as well as 

 in size, and amounts to little more than saying that in one case the 

 organ was enlarged and in the other diminished. 



The claim might be set up that some species of the genus Pro- 

 merycochcerus, especially such a species as P. vantasselensis is to be 

 regarded as an immediate forerunner of Merycochcerus and that through 

 it the phylogenetic history of these two genera is to be explained. 

 Against such an hypothesis is the fact that in P. vantasselensis there 

 are a number of tenaciously persistent characters, which recall those 

 of the earlier types, P. carrikeri and P. chelydra. In P. vantasselensis 

 there is, for instance, no tendency in the superior incisors to become 

 larger, no sudden contraction of the muzzle immediately in front of 

 the jugal, no shifting backward of the infra-orbital foramen, no ap- 

 preciable widening of the upper portion of the occipital plate, and a 

 number of other minor cranial features, which should go hand-in-hand 

 with the progressive changes of the premolar-molar dentition and the 

 shortening of the nasals in order to completely represent the points 

 required in a true ancestor of Merycochcerus (compare Plate XL with 

 Figure 2 on Plate XLII). 



-' Cook, Harold J., "A New Proboscidian {Gamphotherium condon) from the 

 Upper Harrison beds of Nebraska," American Journal of Science (4), Vol. XXVIII, 

 Aug., 1909, pp. 183-184. 



