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 corpiis callosum rudimentary. Heart 

 with two venae. 



Suborder POLYPROTODONTIA. 



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

 ors. Molars with sharp cusps. (Flov^er and Lydekker.) 



Family DIDELPHIID^. 



OPOSSUMS. 



Incisors f . Hind feet with the four outer toes subequal, distinct, 

 and a well-developed opposable hallux. 



Genus DIDELPHIS Linnaeus (17S8). 



DidelpliiK LiNN^us, Syst Nat, 10th erl., I, 1758. p. 54. 



Type. — Didelphis marsupialis Linnajus. 



Arboreal; feet not webbed. 



Dentition. — l.,'iEi\ C., jEi; P-, grg; M.,j5^=50. 



KEY TO THE SPECIES OF DIDELPHIS FOUND ON THE BOUNDARy LINE." 



Tail blacli for basal third ; generally four-fifths the length of head and body or 

 more (tail ratio about 90-95) Didclphis mesamericaiui tcreihiis (p. 1.50). 



Tail blacli only at extreme base ; generally less than two-thirds the length of 

 head, and body (tail ratio about 70) DMclpMs virriiiiiaiia (p. 153). 



DIDELPHIS MESAMERICANA TEXENSIS (J. A. Allen). 

 TEXAS OPOSSUM. 



Didelphis wesamericana texcnsis 3. A. Allen, Bull. Amer. Mus. Nat. Hist, 

 XIV, Art XI, June 15, 1901, p. 172. 



Type-locality.— Brownsville, Cameron County, Texas. 

 Type-specimen.— CAt. No. ||i|f, U.S.N.M. 



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

 ces Bay southward, and the Lower Rio Grande Valley, as far up 



o See Preliminary Study of tlie North American Opossums of the Genu?: 

 Didelphis, by Dr. J. A. Allen, Bull. Am. Mus. Nat. Hist., XIV, Art XI, pp. 149- 

 188, June 15, 1901. 



