122 AMERICAN MESOZOIC MAMMALIA 



The primary cusps are all very slender and high, the conules are well developed, 

 and the pa and me are closely approximated, with a common base, and strongly com- 

 pressed transversely. The pa is slightly higher than the me and the latter is of about 

 the same height as the pr. The external cingulum is pediomyine in form, with a strong 

 metastylar spur but without median stylar cusps. The presence of very narrow ledges 

 on the anterior and posterior sides of the base of the protocone is noteworthy as indicat- 

 ing the first tendency to form cingula here. This piercing type of tooth is quite different 

 in adaptational tendency from those of Pediomys and represents the opposite extreme 

 iYom Didelf/wdon (below). 



Didelphodon Marsh 1889 



1889. Didelphodon, Marsh, Amer, Jour. Set. (3) XXXVIII, 88. 

 1889. Didelfhofs, Marsh, Amer. Jour. Sci. (3) XXXVIII, 179. 



Definition. — Based on didelphodontine upper molars with stylar cusp B well 

 developed, larger than the paracone. No median outer cusp. D small. E large and 

 ridge-like. 



Type. — D. vorax Marsh. 



Distribution. — Lance, Wyoming. 



Besides the genotype, Marsh assigned two other species to this genus. D. jerox 

 was based on a badly worn and broken lower molar. Reference to this genus does not 

 rest on any direct evidence. D. jerox is synonymous with "Cimolestes" curtus, but the 

 latter species does not belong in the genus Cimolestes. D. comfhis was also based on an 

 isolated lower molar. It is not congeneric with D. jerox. These lower teeth are treated 

 below. For the present the name Didelphodon must be confined to upper molars di- 

 rectly comparable with the genoholotype. The two associated upper molars referred to 

 D. vorax ( Marsh 1 892A, PI. IX, fig. i ) belong to Pediomys hatcheri. 



The name Didelfhodon was changed to Didelfhofs by Marsh on the basis of 

 essential preoccupation by Didelfhodus. Such changes are sanctioned, or even de- 

 manded, by many writers and the matter may be one of personal opinion. Unfortu- 

 nately the I nternational Rules of Zoological Nomenclature deal with the matter only 

 in a recommendation,'" but this recommendation the writer prefers always to follow. 

 Such action certainly makes for greater stability in zoologic nomenclature as a whole, 

 and can seldom lead to any real confusion. When two names of closely related animals 

 differ only by one or two letters and cannot be considered as homonyms under Art. 35, 

 special action may occasionally be necessary, but not by a single worker. 



'Didelfhodon vorax Marsh 1 889 



1889. Didelfhodon vorax, Marsh, Amer. Jour. Sci. (3) XXXVIII, 88. 

 1892. Didelf hops vorax, Marsh, Amer. Jour. Sci. (3) X LI II, 262. 



'° Under Art. 36. "... When once introduced such names are not to be rejected on this account. 

 Examples : . . . Polyodus, Polyodon, ..." 



