MORRISON MAMMALS: PANTOTHERIA 57 



even if only temporary. On the other hand, it was agreed that names of more than 

 generic rank should not be based on unassociated upper jaws. This, also, has been done 

 in the past but has proven often to have very unfortunate results. 



In accordance with this decision six genera have been established on upper jaws: 

 Herpetairus, Melanodon, Malthacolestes, Eutlilastus, Miccylotyrans, and Pelicopsis. 

 These are all provisionally referred to the Dryolestidae. In the cases of Herpetairus, 

 Melanodon, Eut/tlastus. and Miccylotyrans, this reference is supported by much direct 

 and indirect evidence and is probably correct. Malthacolestes and Pelicopsis, on the 

 other hand, very possibly are not dryolestids. They may be paurodontids, or their rare 

 remains may possibly represent genera or other groups otherwise unknown. In view of 

 this doubt it seems best to associate them with the other upper jaws, with this word of 

 caution. None of them are docodontids; the peculiar upper teeth of this family are well 

 known and are referable to the genus Docodon with little possibility of error. 



As here classified, the generic divisions within the family Dryolestidae are based 

 primarily on molar structure. They are so drawn that a single unbroken and little 

 worn molar may be referred to its correct genus in any given instance. Bearing in 

 mind the fact that it is often impossible to distinguish between closely related Tertiary 

 and recent genera from single teeth, this procedure has possibly resulted in drawing 

 generic lines rather more broadly than is often done, but such a result requires no 

 apology. It is more useful, at least in the present case, and does not greatly modify the 

 modern concept of a genus. 



Dryolestes Mzxsh. 1878 



1878. Dryolestes, Marsh, Amer. Jour. Sci. (3) XV, 459. 

 1887. Asthenodon, Marsh, Amer. Jour. Sci. (3) XXXIII, 336. 



Definition.— Dental formula I4 Ci P4 Mg. Premolars with discontinuous inter- 

 nal cingulum, with distinct anterior accessory cusp. Metaconid of molars columnar, 

 not bifid; paraconid slightly procumbent, lower than me-^; pa'', me'', and talonid cusp 

 in a straight longitudinal line. No external cingulum. 



Type. — Dryolestes priscus Marsh. 



Type of Asthenodon. — Asthenodon segnis Marsh = Dryolestes priscus Marsh. 



Distribution. — Morrison formation, Wyoming. 



As mentioned again below. Marsh's figure of "Dryolestes priscus" (1887, PI. I X, 

 fig. 2) was not, as has since been supposed, of the genoholotype but of a referred speci- 

 men which proves on further study to belong to a distinct genus. Even more curious 

 and unfortunate is the history of the supposed genus "Asthenodon." Asthenodon was 

 differentiated from Dryolestes by Marsh on the more uniform size of the tooth series 

 and the presence of three, rather than four, premolars. Careful measurement, after 

 more complete preparation, shows that the teeth do not differ from Dryolestes in size. 

 The supposed presence of but three premolars was also a very natural error. In the 

 t>pe of Asthenodon segnis, the first tooth is not a canine, as first stated, but a premolar 

 which had become loose and was unnaturally far out of the alveolus when buried. 



