150 BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



Order MARSUPIALIA. 



MARSUPIALS. 



Young at birth are usually placed in an abdominal pouch formed 

 by a fold of skin about the milk glands of the mother, where they 

 remain for a considerable time. Reproductive organs of peculiar 

 structure in both sexes, nearly all the parts being double in the 

 female. Brain small, the corpus callosum rudimentary. Heart 

 with two vena*. 



Suborder POLYPROTODONTIA. 



Incisors numerous, small, subequal. Canines larger than the incis- 

 ors. Molars with sharp cusps. (Florrcr and Lydekker.) 



Family DIDELPPIIID.E. 



OPOSSUMS. 



Incisors 5. Hind feet with the four outer toes sul^eipial, distinct, 

 and a well-develo})ed opposable hallux. 



Genus DIDELPHIS Linnseus (1758). 



Dhlclphix LiNN.TCUs, Syst. N;it.. Kith (h1.. I. 17r>S. ]>. .">4. 



Type. — Dldelphis mavHiiphdix Linna'us. 



Arboreal; feet not webbed. 



Dentition.— I., ''^^■, C, JEJ; I*-, K; M.,j5|=50. 



KKY TO THE SPECIES OF DIDELPHIS FOUND ON THE BOUNDARY LINE." 



Tail black for basnl third ; <;ener:ill.v fonr-fifths the length of head .and body or 

 more (t;iil ratio about 00-!15) l>i(1cli)liis DiCHdiiicricaiid ir.misis (jt. ir>0). 



Tail black only at extreme base; senerally less than two-thirds the leiitrth of 

 head and body (tail ratio about TO) ])i(lrii)]ii.-< rirfiiiiiniKi (]). l."."!). 



DIDELPHIS MESAMERICANA TEXENSIS (J.A.Allen). 



TEXAS OPOSSUM. 



Didrlpliis mcaameyicunn texciisis .]. A. Ai.i.kn. Rnll. Anier. Mns. Nat. Hist., 

 XIV, Art. XI. .Tune 1.5, 1901, p. 172. 



Type-loc(dity. — Brownsville, Cameron County, Texas. 

 Type-i<peciinen.—OAi. No. |fH|, U.S.N.M. 



Geographical distribution. — The coast region of Texas, from Nue- 

 ces Bay southw^ard, and the Lower Rio Grande Valley, as far up 



« See Preliminary Study of the North American Opossums of the (Jcnn^; 

 Didelphis. by Dr. .1. A. Allen. Bull. Am. Mus. Nat. Hist, XIV. Art. XI, pj). 140- 

 188, June 15, 1901. 



