478 THE MAMMALIA OF THE UINTA FORMATION-. 



The chief interest wliieh attaches to Plesiarctomys lies in the light which it thi'ows 

 npon the difficult problem of the origin of the rodent dentition. As to this question 

 Schlosser says : " Die Ziihne dieser altesten Formen erinnern einigermassen an das 

 Omnivoren-Geh\&s. Doch lasst sich die urspriingliche Beschaffenheit nicht mehr 

 erraitteln. "Wahrscheinlich bestand der einzelne Backzahn aus sechs Hiigeln 

 ('Denticnles '), deren je drei in einer Zahnhalfte vorhanden waren. Der Typiis eines 

 solchen PriV/i/iM'-Omj/it'oreji-Zahnes ist noch am besten in den oberen Molaren von 

 Diclwhune erhalton. Hier tiitt nicht selten noch der sechste Hiigel auf, worauf schon 

 Kowalevsky aufmerksam gemacht hat. Bereits zu Anfang der Eocaenzeit erscheint 

 eine grosse Anzahl verschiedener Niiger, die indess nur insofei-ne unter einander ver- 

 wandt sind, als sie mit Ausnahme von ganz wenigcn Arten — Mijomoiyha — als Mittel- 

 formen zvvischen den in der Jetztvvelt ziemlich scharf getrennten Uystncomorphen 

 und Sciumorphen angesehen werden miissen. Auch fehlen keineswegs Mittelformen 

 zwischen diesen and den Myomoiyhen'''' {'Eo. 14, pp. 98-99). After a further discus- 

 sion of the subject Schlosser concludes that the rodents are to be derived from the 

 miirsupials. 



Plesiarctomys lends but little snppoit to these views, and this genus is entitled 

 to special consideration as being one of the oldest if not the very oldest known form 

 of rodent. Here the superior molars are plainly of the tritubercular pattern, forming 

 distinct crests only in the later species. The inferior molars show the anterior trian- 

 gle of three cusps with a talon behind, or what Cope has termed the tuberculo-secto- 

 rial molar. This is the type of dentition which is all but universal in the Puerco 

 fauna, ungulates, creodonts and lemuroids all exhibit it, and Plesiarctomys seems to 

 show that the rodents are to be derived from the same generalized group of primitive 

 placental mammals, the Bunotheria, to which Ave refer the origin of the types just 

 mentioned. It is, to say the least, very suggestive that the most ancient known 

 types of rodents should exhibit the tritubercular plan of molar structure so unmis- 

 tably, though it must he admitted that this of itself is not conclusive against their 

 derivation from the marsupials. It removes, however, the necessity of such a deriva- 

 tion, and the other reasons assigned by Schlosser for his view do not appear to be 

 very cogent. 



Plesiarctomys is, on the whole, to be regarded as a member of the Sciumorpha, 

 but cei'tain features, such as the lai'ge infraorbital foramen and the absence of i)0st- 

 orliital processes, are jjoints of resemblance to the Hystricomorpha. This association 

 of characters, as is shown in the passage already quoted from Schlosser, is very gen- 

 erally found to occur in the Eocene rodents. 



