1 90 1.] Allen, Opossums of the Genus Didelphis. 177 



where, however, the black form prevails; and also in D. k. caucce, 

 where the two phases are about equally represented. It has 

 heretofore been very difficult to account, for example, for the 

 occurrence of black and gray specimens among the very young 

 specimens as well as among the adults, but from this point of 

 view the matter becomes very simple. The type of D. cauca, it 

 may be added, is a specimen in the gray phase, while the type of 

 D. colombica is a worn specimen in the black phase. 



EXPLANATION OF THE TABLES OF MEASUREMENTS, ETC. 



All the measurements are in millimetres. In measuring skulls 

 the total length is taken from the front border of the premaxil- 

 laries to the posterior border of the occipital, crest. In young 

 animals, which have just acquired m^-, this is often less than the 

 basal length, but in animals more advanced in age, and especially 

 in very old individuals, it exceeds by about 10$ the basal length. 

 Basal length, in the present paper, is the basal length of Hensel, 

 and is taken from the inner base of the upper incisors to the front 

 border of the foramen magnum. The other skull measurements 

 require no special explanation. 



In all skulls of which measurements are tabulated the fourth 

 molar has reached its full development. . In the ' sex and age ' 

 column ad. y. = young adult; ad. m. = middle-aged adult; and 

 ad. o. == very old adult. 



The external measurements are in every case those taken by 

 the collector from the fresh specimen. In most cases only three 

 measurements -appear to have been taken, namely, the total 

 length, the length of the tail, and the length of the tarsus. For 

 purposes of comparison, and in the computing of ratios, it has 

 been found convenient to separate the length of the head and 

 body, which is found by subtracting the tail length from the 

 total length. 



The source of the material used is indicated by prefixing to the 

 numbers the letter A to distinguish the specimens from the 

 American Museum of Natural History. All those not thus dis- 

 tinguished are from the Biological Survey Collection in the United 

 States National Museum, Washington, D. C. 



In the case of fractional numbers, the number above the line 

 refers to the skull and the number below the line to the skin. 

 \June, igoiJ\ 12 



