460 



one of which becomes the A. vesicalis, the other the A. pudenda. 

 The distribution of this artery is the same in Didelphys as that 

 described for Echidna. In a foot-note Hochstetter states that 

 the two branches of this artery may arise as separate vessels. I 

 found this to be the case in several instances in Didelphys, either 

 for one or both sides. See Figs. 3 and G (right side). The A. vesi- 

 calis is not labelled in Fig. 6. 



In connection with this double origin of the A. pudendo-vesi- 

 calis, it is interesting to note that, in two instances, the vein that 

 corresponds to this artery was also found to be double 

 in Didelphys, Fig. 3 (dextra). 



What the causes may be that are responsible for the production 

 of such a constant variation as that described above, it is impossible 

 to say. The following quotation is certainly most interesting in this 

 connection. 



In referring to Didelphys marsupialis, var. typica, 

 Oldfield Thomas says^): "This wide spread species, owing to its 

 remarkable variability in color, has been made the basis of a very 

 considerable number of nominal species, of which the most commonly 

 recognized are the North American D. Virginian a, the Brazilian 

 D. cancrivora, and the striped-faced D. azarae. I find, 

 however, such a considerable amount of variability 

 in the specimens from every locality, and such an 

 entire absence of constancy in any character or set 

 of characters, that I am constrained to unite the whole 

 of this group of Opossums into a single species, to 

 which the Linnean name D. marsupialis is of course 

 applicable"-). 



Princeton, New Jersey, U.S.A., July 1900. 



1) Catalogue of the Marsupialia aud Monotremata in the Collection 

 of the British Museum (Natural Histoiy), 1888, p. 327. 



2) The spacing was added by the writer. 



