NO. IGSO. THE FOSSIL GENUS PTILODUS—aWLEY. 523 



grarilh were obtained, represents a third and much smaller species of 

 Pt'dodiis from these beds. This tooth is about the size of the type of 

 /'. tf<)ii('ss((rf 'hoiks Cope, but does not agree with it in proportions, 

 the latter being- relatively much lower crowned. In this respect it 

 corresponds much more closely and agrees also in size with the type 

 of " Ilalodon serratus " Marsh, from the Ceratops beds of Converse 

 Count}^, AVyoming. For the present I provisionally i-efer it to 

 Marsh's species, although there are minor dift'erences which indicate 

 that it may later be placed in a new species. 



PTILODUS FORMOSUS? <• (Marsh). 



A fourth and still smaller species is represented in this collection 

 from the Fort Union by two lower fourth premolars (Cat. Nos. G089 

 and (')090, U.S.N.M.). These teeth, which are only 3.2 mm. in 

 length, agree in size, proportions, and the number (12) of enamel 

 ridges with Marsh's " Ilalodon formosus^'' from which they can not 

 at present be distinguished. The type of this species is also from the 

 Ceratops beds. 



ZOOLOGICAI. RELATIONS. 



Owing to the absence of good material, well-defined characters other 

 than those presented in the teeth have hitherto been wanting. In 

 conseijuence there has been a wide diversity of opinions regarding 

 the relation of the Multituberculates to other great groups of the 

 Mammalia. Earlier writers, studying Jurassic forms, classed them 

 with the Marsiij)i(di<i. Marsh, in 1S80,'' first proposed placing the 

 group in a distinct order, which he named the Ailothcria^ although 

 recognizing their marsupial affinities. In defining the order he said : " 

 " These characters alone do not indeed separate the Plagiaulacidw 

 from some of the Marsupials, and future discoveries may prove them 

 to belong in that group, where they would then represent a well- 

 mnrked suborder." Later Cope '" proposed the name MvltHvljer- 

 ciilata for this same grouj), which he considered a suborder of the 

 Marsujnalia. Still later he suggested their relationship to the Mono- 

 tremes. At present most authorities rank the group as a full order, 

 which is variously classed with the Marsupials or the Monotremes. 



As has l)een stated by Osborn,'' the relationship of the group to the 

 Marsupialia, which was first proposed by Falconer, had not been 

 questioned until the discovery, by Poulton, of the early-shed multi- 

 tubercular teeth of Orrdfhorhynedius. This led Cope to suggest the 

 reference of the Multituberculates to the M onotremata^ a view which 



'^Ilalodon formosus Mai'sli, Anier. Journ. Sci., XXXVIII, 1S.S9, p. 171), pi. 

 VIII, fig. 36. 



''Amer. Jour. Sci., (3) XX, 1880, p. 230. 



''Amer. Naturalist, XVIII, 1884, p. 087. 



<^Tlie Structure and Classificatiou of the Mesozoic Mammalia, ,7ourn. Acad. 

 Nat. Sci., riiila., (2), IX, 1888, p. 254. 



