I20 



AMERICAN MESOZOIC MAMMALIA 



oblique, with a pronounced external spur, but in this case the spur is parastylar, not 

 metastylar as in M^"*. A forms the end of this spur. B is absent, C slightly developed. 

 D and E are absent and the cingulum is obsolete posteriorly. The metacone is reduced 

 and the metaconule approximated to it. 



Skull 



iw.tc 



F.PPI. 



RN?. 



r.WR, 



YP-M-I069£ 



Fig. 44. Pediomys sp. Left half of palate. 

 Superior and inferior views, ij^ times 

 natural size. 



F.P.P.E., bristle passing through pos- 

 teroexternal palatine foramen. F.F.P.R., 

 bristle passing through small foramen in 

 postpalatal ridge. F.S.P., sphenopalatine 

 foramen. IN.F.C, floor of infraorbital 

 canal. O.F., floor of orbit. P.N.P., poste- 

 rior narial passage. P.V., edge of palatal 

 vacuity. 



Various small fragments of the skull of 

 this genus are known, the most important being 

 Y.P.M. No. 10692 (see Fig. 44), the posterior 

 left portion of a palate without teeth. Its chief 

 interest lies in its close agreement with the re- 

 cent Didelfhis. The structure is well shown in 

 the accompanying figure and has no feature 

 which may not be found in the recent opossum. 

 This is the specimen to which Marsh had refer- 

 ence ( 1889B, p. 179) when he suggested that the 

 rounded margins of the vacuity might indicate 

 that it was functional as the posterior nares, but 

 the vacuity is equally well rounded in many 

 recent specimens, and it is inconceivable that it 

 can ever have had this function. 



Nyssodon'$)iva^son 1927 



1927. Nyssodon,SimTpson,Amer. Jour.Sci. (5) XIV, 

 124. 



Definition. — Primary cusps of typical 

 upper molar all slender and high, pa and me closely approximated, with connate bases. 

 Narrow anterior and posterior cingula on base of pr. No median stylar cusps. 



Type. — N. functidens Simpson. 



Distribution. — Lance, Wyoming. 



This genus is obviously quite distinct from anything that could be referred to 

 Pediomys. There is a possibility that it is not a didelphid, but in view of the abundance 

 of didelphids in the Lance and of a certain fundamental resemblance to Pediomys it 

 may be considered as a pediomyine at least until it is better known. 



Nyssodon functidens Simpson 1927 

 1927. N. functidens, Simpson, Amer. Jour. Sci. (5) XIV, 124. 



Type. — Y.P.M. No. 13654. Isolated upper molar. 



Horizon and Locality. — Lance, Niobrara County, Wyoming. The tj^e is from 

 Hatcher's Quarry 9 mammal locality, see Lull 191 5a. 

 Diagnosis. — Sole known species of the genus. 



