448 



REPOET OF STATE GEOLOGIST. 



tachment of the embryo to the uterus and the young are born in a 

 very rudimentary condition. 



The earliest mammals, geologically, of which we have any knowl- 

 edge are marsupials. The lack of specialization in some members 

 of the order has enabled them to become adapted to changing con- 

 ditions and they have survived with very few changes since Miocene 

 times. The order is now limited in distribution to America and 

 Australia. 



Family DIDELPHIDAE. 



' OPOSSUMS. 



This family includes all of the marsupials of North and South 

 America, to which continents it is limited. It is characterized by 

 the presence of 50 teeth, five toes on each foot, the first toe of the 

 hind foot being wddely separated from the rest, and a long scaly, 

 prehensile tail. 



Genus Didelphis Linnaeus. 

 Dental Formula.— 1, f^; C, \^]; Pm, 'l^; M, |^ = 50. 



Generic cJiaracters. — A complete marsupium or jxjuch on the 

 abdomen of the female in which the young are placed immediately 

 after birth; young born at a very early stage when development is 

 less advanced than in most mammals. Tail prehensile, densely 

 haired at the base but nearly naked on the outer two-thirds. Inner 

 toe of hind foot somewhat opposable to other toes. Five digits on 

 both fore and hind feet. 



Skull elongated, with a high sagittal crest, pointed rostrum, and 

 numerous teeth. 



The genus has a range extending from the northern United 

 States to Middle South America and is represented by a number of 

 species. Only a single form occurs in the eastern United States 

 north of Florida. 



1. DIDELPHIS VI KG INI AN A Kerr. 

 OPOSSUM. 



Didelphis virginiana Kerr, Animal Kingdom, p. 198, 1792. 

 Evermann and Butler, Proc. Ind. Acad. Sci. for 1893, p. 

 124, 1894. 



Didelphis mMrsitjnalis Miller and Relui, Proc. Post. Soc. Nat. 



Hist., p. 7, 1901. 

 Diagnostic characters. — Easily distinguished from any other 

 animal found in the State by the marsupium of the leriiale, the op- 



