UPPER CRETACEOUS MARSUPIALIA 117 



The Pediomyinae include the more primitive Cretaceous forms and probably 

 some or all of the Paleocene ones. This is the most primitive subfamily and apparently 

 represents the structural ancestry of the whole family, or indeed of the whole order. 

 The Didelphodontinae are known only from the North American upper Cretaceous 

 and represent an aberrant group of durophagous adaptation. The Microbiotheriinae 

 are confined to the Tertiary of South America. Although often given a very central 

 position, as by Ameghino, they are probably somewhat aberrant and are probably not 

 directly ancestral to any living didelphines, which they seem to resemble rather less 

 than do some of the Pediomyinae. The subfamily Didelphiinae includes all of the liv- 

 ing opossums and the Tertiary forms except the microbiotheriines. It is, on the whole, 

 more primitive than either Microbiotheriinae or Didelphodontinae and possibly is 

 directly descended, with some modification, from the Pediomyinae. 



In the Cretaceous the Pediomyinae and Didelphodontinae are represented, but at 

 present it is impossible to assign all of the named genera to one or the other family. 

 The following are surely members of the Pediomyinae : Pediomys, Nyssodon. The fol- 

 lowing surely belong in the Didelphodontinae: Didel-phodon, Ectoconodon, Thlaeo- 

 don, Alfhadon, Stagodon. Eodelfhis and Enangelistes include only lower teeth as yet, 

 but are probably pediomyine, and Diaflwrodon is closely similar to Eodelfkis. Cimo- 

 lestes, Delfhodon, and the unnamed types of lower teeth are of doubtful affinities. 



A, GENERA INCLUDING UPPER MOLARS 



Pediomys Marsh 1889 



1889. Pediomys, Marsh, Amer. Jour. Sci. (3) XXXVIII, 89. 



Definition. — Upper molars pediomyine, molar cusps relatively low. Pa and me 

 well separated. No prominent cingula on base of pr. 

 Type. — P. elegans Marsh. 

 Distribution. — Lance, Wyoming. Hell Creek, Montana. 



Included Genus 



1898. Protolambda, Osborn, Bui. Amer. Mus. Nat. Hist., X, 172. Type: P. hatcheri Osbom. 



This genus, sensu lato, is a very large one, including the greater number of Lance 

 didelphids. Two or more genera may be included, although if so they are very closely 

 related. Certainly there are a number of species, but none can be usefully defined at 

 present. Protolambda Osborn is based on a tooth larger than the type of Pediomys ele- 

 gans and his specimen may be retained as specifically distinct, although it shows no 

 distinctions of true generic character. The original diagnosis and figure did not sepa- 

 rate Protolambda from Pediomys except for the fact that the types of the former are 

 labeled as if from the left side, although all three are really right upper molars. 



