56 AMERICAN MESOZOIC MAMMALIA 



has naturally resulted in a considerable decrease in the number of genera. Among 

 the genera earlier based on lower jaws, only Dryolestes, Amblotherium, Perasfalax, 

 and Phascolestes seem to be valid. Two genera have been added by the author. One, 

 Laolestes, was previously included under Dryolestes which it closely resembles, and 

 the other, Ke-polestes, comes from a hitherto unmentioned locality for Jurassic mam- 

 mals. 



Perhaps the most gratifying result of the reexamination of the magnificent Marsh 

 Collection, however, has been the discovery of numerous pantothere upper jaws. A few 

 fragments were known from the Purbeck beds, the best described in detail by Osborn 

 under the name of Kurtodon. Marsh had named two upper jaws from the Morrison as 

 species of Dryolestes, and Osborn (1904) had given accurate figures of two upper jaws, 

 including one of Marsh's types, but unfortunately this material was, from a morpho- 

 logical point of view, the worst in the whole collection, with the molars badly worn 

 and much broken. The only figure or detailed description of an adequately preserved 

 Morrison pantothere upper molar was that of Gidley (1906, PI. V, figs. 2-3). The 

 apparent absence of such material was justly lamented, but when the Marsh Collection 

 was overhauled, it was found that a large number of upper jaws, many still concealed 

 in the matrix, had been set aside by Professor Marsh for future study. As will appear 

 below, not only was the number of specimens surprisingly large, but they exhibited a 

 most unexpected and welcome morphological variety. 



The cataloguing and description of this material has involved a nomenclatural 

 difficulty. I n no case is there any association between these extremely important upper 

 jaws and the lower jaws on which the whole nomenclature of the group has hitherto 

 been primarily based. In the case of the triconodonts and docodontids, for instance, 

 this lack of association is not serious, for here a study of occlusal relationships permits 

 reasonably certain inference as to the taxonomic position of the upper jaws, but in 

 dealing with the dryolestids this method also is unavailable. I n no case are the pro- 

 portions, dimensions, and occlusal relationships such that a definite type of upper 

 molars could be referred with high probability to one genus of lower jaws and only 

 one. Arbitrary reference to established genera would imply knowledge where none 

 exists and, even if correct in some cases, would certainly be erroneous in others and 

 would lead to almost ineradicable errors. Description of this fundamentally important 

 material, — material which must inevitably gain an important place in many discussions 

 involving the fundamental characters and early history of the Class Mammalia, — 

 without applying names to it seems highly undesirable, if only from the point of view 

 of convenience, which is, after all, the primary reason for the use of names. The question 

 seemed of such importance that the advice of several eminent students of fossil mam- 

 mals was sought and freely given. All agreed that the science would be best served by 

 comparing the upper jaws carefully, dividing them into what appear to be natural 

 genera, and applying new names to these genera regardless of the fact that some of 

 these new names may, if associated specimens are found, be found to be synonymous 

 with older names already applied to lower jaws only. There are excellent precedents 

 for this course and it has proven to be entirely justified in the past by its usefulness, 



