XOTES OX THE FOSSIL INIAMMALIAX GENUS PTILODUS, 

 AVITH DESCRIPTIONS OF NEW SPECIES. 



By Jajiks Williams Gidley, 

 Custodian wf Fossil Mainnial.s, I'nifed States yatiiinal Miisrum. 



INTRODUCTION. 



The recent fortunate discovery of a fine specimen of Pt'dochu^, in 

 Avliich the lower juAvs and some other parts of the skeleton are asso- 

 ciated with a nearly complete skull, not only adds materially to our 

 knowled^'e of this genus, ])ut makes possible the clearing iq) of 

 much of the confusion still existing regarding the classification and 

 relationships of the entire Multitnberculate (Allotheria) group. 



In view of the very fragmentary material hitherto representing 

 this great group of mammals tliis new material is of more than usual 

 interest and importance. The comparative completeness of the speci- 

 men i^ermits for the first time a fairly accurate study of the mor- 

 l)hology of the little creature, and makes it possible to determine its 

 relationshiiDs with more certainty than has hitherto been done. 



This interesting specimen represents a new^ species which is de- 

 scribed below. It is from the Fort Union beds of Sweet Grass 

 County, Montana, where it was found by Mr. A. C. Silberling in the 

 spring of 1908 while collecting fossil mammals in that locality under 

 a special grant from the United States Geological Survey. In this 

 and subsequent collections obtained by Mr. Silberling through a 

 continuation of the work by the U. S. National Museum, there are 

 disassociated upper and lower jaws of several other individuals 

 of this and a second species of the genus. The greater part of this 

 collection, however, consists of numerous specimens, mostly teeth and 

 jaws, representing a large number of genera and species of Basal 

 Eocene mammals, many of them closely related to or identical with 

 species from the Torrejon of New Mexico. Several of the forms are 

 apparently new, and Avhen studied will be published. 



Before describing this new species of Ptilodns it seems desirable to 

 first give a short history of the genus and a summary revision of the 

 Torrejon and Fort Union species already referred to it. 



Proceedings U. S. National Museum, Vol. XXXVI— No. 1689. 



611 



