1901.] Allen, Opossums of the Genus Didelphis. 187 



EXPLANATION OF THE PLATES. 

 PLATE XXII. 



VARIATION DUE TO AGE IN DIDELPHIS VIRGINIANA. 

 Figures all \ nat. size. 



Fig. i. No. 48598, U. S. Nat. Mus., $ , Cedar Vale, Kan. Side view. 



Fig. la. Same specimen from side. 



Fig. i. Same specimen, from below. Milk premolar and m^ in place, 



Fig. 2. No. 47498, U. S. Nat. Mus., ? , Cedar Vale, Kan. Side view. 



Fig. 2a. Same specimen, from below. Milk premolar and m 1 and m^ in 

 place. 



Fig. 3- No. 49364, U. S. Nat. Mus., $ , Brazos, Texas. Side view. 



Fig. 3#. Same specimen, from below. The milk premolar has been shed 

 and p^ is just appearing ; m 1 " 5 - in place, the latter about half grown. 



Fig. 4. No. 79117, U. S. Nat. Mus., $ , Sebastian, Florida. From side. 



Fig. 40. Same specimen from below. Permanent p- and m^ nearly full 

 grown ; nr 4 not yet developed. 



Fig. 5. No. 45401, U. S. Nat. Mus., $ , Bay St. Louis, Miss. From side. 



Fig. 5. Same specimen from side. Fully adult, but teeth very little 

 worn. 



This specimen is of further interest on account of the double canine on the 

 left side. 



PLATE XXIII. 



DIDELPHIS MARSUPIALIS, D. M. TABASCENSIS, AND D. VIRGINIANA. 

 All figures ^ nat. size, except figs. 6 and 7, which are natural size. 



Fig. i. Didelphis marsupialis tabascensis, ? ad., No. 100509, U. S. Nat. 

 Mus., Teapa, Tabasco, Mexico. 



Fig. 2. Didelphis marsupialis tabascensis, $ ad., No. 100513, U. S. Nat. 

 Mus., Teapa, Tabasco, Mexico. Figures i and 2 illustrate individual varia- 

 tion in the form of the posterior border of the nasals. 



Fig. 3. Didelphis marsupialis, $ ad., No. 45274, Colima, Colima, 

 Mexico, showing the usual or normal form of the nasals in D. marsupialis, for 

 comparison with Fig. 2, which illustrates the normal form of the nasals in D. 

 m. tabascensis, and Fig. 4, which shows the normal form of the nasals in D. 

 virginiana, 



Fig. 4. Didelphis virginiana, $ ad., No. 316, Miller's Place, Long Island, 

 N. Y., illustrating a normal old male skull, for comparison with the other forms 

 of the genus. 



Fig. 5. Didelphis marsupialis, $ ad., No. 73492, U. S. Nat. Mus., Te- 

 huantepec, Oaxaca, Mexico. To show the pair of supernumerary molars (m^). 



Fig. 6. Didelphis marsupialis tabascensis, $ ad., No. 77688, U. S. Nat. 

 Mus., Huehuetan, Chiapas, Mexico. To show supernumerary m^ on left side. 



