Peterson: The Osteology of Promerycociicerus. 219 



Although speculation along such lines as these is alluring, and some- 

 times irresistible to the student, anatomical features, which are at 

 best rather difficult to understand, even when we have an abundance 

 of perfect material representing the various closely related forms, 

 should not be confidently regarded as supporting theories, which at 

 best often prove fallacious. 



Finally attention should be called to the fact that there have been 

 already established two genera, Merycochoerus and Pronomotherium, 

 which are quite variant, and represent two widely diverging races or 

 phyla. It is the view of the writer that Promerycochcerus represents 

 an additional phylum of the large oreodonts, which occurred in the 

 Miocene. It has been stated elsewhere that the type of the imperfectly 

 known form, Merychyus major Leidy, has the infra-orbital foramen so 

 situated as to recall P. vantasselensis. Judging from the characters 

 observed in the latter species, it belonged to a phylum which undoubt- 

 edly had a high and narrow occiput, moderately long cranium, short 

 face, and the side of the face divided into two fossae, which features 

 are also found in smaller forms, such as Merychyus, and are survivals 

 of characters found in earlier Oligocene forms, while other features 

 represent a development along lines more nearly parallel with what 

 we see in Merycochosrus. Whenever more nearly complete remains 

 of Merychyus major from the Middle Miocene are discovered, it will 

 probably be shown that 4;his species belongs more nearly to the line 

 of P. vantasselensis. The immediate ancestors and the manner of the 

 evolution of Merycocha'riis and Pronomotherium are as yet imperfectly 

 known. 



The sudden appearance of new forms in a given geological horizon 

 no doubt frequently indicates that they are immigrants in the locality 

 where they occur, and represent lines of evolution parallel to that of 

 the indigenous forms with the remains of which they are intermingled. 

 The species of Promerycochcerus found in the Lower Miocene of 

 Nebraska and Wyoming possibly may be descended from such forms 

 as P. chelydra from the John Day beds (Compare Plates XXXVIII 

 and XL with Plate XLI and Figure i of Plate XLII); their descent 

 is not, so far as we now know, traceable from any of the Oligocene 

 forms hitherto found in the same general region in which they occur. 

 The smaller Oligocene Oreodonts appear on the other hand to have 

 their descendants in such forms as Eporeodon, Mesoreodon, Phena- 

 cocxlus, Merychyus, and finally in the Pliocene Metoreodon. 



