ii6 AMERICAN MESOZOIC MAMMALIA 



DIDELPHIIDAE Gray 1821 



Definition.— Dental formula Itl C\ Pi M|. Incisors small. Canines large. Pre- 

 molars simple, trenchant or bulbous. Lower molars tuberculo-sectorial, with three 

 trigonid cusps and basined heels with hypoconulid posterointernal, near entoconid. 

 Upper molars tritubercular, trigonal, with no true hypocone. External cingulum 

 broad, shelf-like, with various well-defined external marginal cusps. Quadrupedal, 

 plantigrade, usually with opposable hallux and pollex, often with prehensile tail. 



Type. — Didelfhis Linnaeus. 



Distribution. — Known : Cretaceous, North and South America. Paleocene, Eu- 

 rope and North America. Eocene and Oligocene, North and South America and 

 Europe. Miocene, South America and Europe. Pliocene, South America. Pleistocene 

 and Recent, North and South America. Inferred: Probably world-wide (save perhaps 

 in Africa) in Cretaceous. In America surviving to the present time. In Europe becom- 

 ing extinct at the end of the Miocene. In Asia becoming extinct at some unknown time. 

 In Australia becoming modified in the early Tertiary into the families typical of this 

 continent. 



The Cretaceous marsupials were apparently all intended by Marsh to be referred 

 to his family Cimolestidae. This family has not been defined and the type genus was 

 based on a lower molar. It would be possible to define the family on the basis of re- 

 ferred upper teeth, but it is poorly known and it certainly differed very little from the 

 Didelphiidae. For this reason the family Didelphiidae is used in a broad sense and is 

 made to include all the primitive opossum-like fossils of North and South America 

 and Europe, not only the Cimolestidae but also the Microbiotheridae of Ameghino. In 

 so far as they are clearly recognizable as distinct groups, the more distinctive of the fos- 

 sil forms may be placed in separate subfamilies, the number and arrangement of which 

 will naturally be altered by further discoveries. At present four subfamilies are clearly 

 distinguishable : 



1. Pediomyinae. Premolars trenchant. Upper molars didelphine in contour. 

 Paracone and metacone subequal. Cingulum on M^'* narrow anteriorly, oblique, pro- 

 longed posteriorly into a sharp metastylar spur. Paraconule definite, connected by a 

 slight crest with the anterior style. Second style absent or weak, fourth larger than 

 third (median). 



2. Didelphodontinae. Premolars bulbous, crushing. External cingulum of upper 

 molars uniformly broad, bilobed, nearly symmetrical, without marked metastylar or 

 parastylar spur. Paracone and metacone subequal. Second style large and heavy, others 

 variable. 



3. Microbiotheriinae. Premolars trenchant. On M^'^ metacone somewhat larger 

 than paracone. No distinct metastylar spur. External cingulum moderately developed, 

 of about equal width throughout, styles not strong. 



4. Didelphiinae. Premolars trenchant. On M^"* paracone much reduced. Cingu- 

 lum strong, produced into a marked metastylar spur. Second style large, fourth 

 smaller than third. Paraconule reduced or absent, not connected with anterior style. 



