Allen, Opossums of the Genus Didelphis. 157 



the space on the alveolar border between the last molar and the 

 front edge of the coronoid, a change occurs in the angle made by 

 the coronoid with the horizontal portion of the ramus, the angle 

 formed by the two axes becoming gradually more open or wider, 

 due to the increased backward slope of the coronoid. This is a 

 fact worthy of attention in comparing young adults with middle- 

 aged or very old specimens. There is also often great individual 

 variation in the slope of the coronoid in specimens of comparable 

 age from the same locality. 



Nasals. 



Normally the nasals are narrow apically for a little more than 

 half their length ; then they abruptly expand, the expanded por- 

 tion taking the form of a rhomboid, being diamond-shaped in 

 outline; the expanded portion occupies a little less than one half 

 the total length (about 42 to 48$). The transverse axis of this 

 expanded portion varies from 75 to 100$ of its longitudinal in 

 different specimens from the same locality. The posterior half 

 of the expanded portion may be pointed and symmetrical in out- 

 line, but usually it is shorter than the anterior half, and not 

 infrequently the posterior border is abruptly truncated. The 

 narrow anterior portion is of nearly equal width throughout, but 

 the breadth varies in individuals from the same locality, the 

 variation amounting to about 30$ of the mean. (See PI. 

 XXIII, Figs, i and 2.) 



The ratio of the nasals to the basal length of the skull varies 

 in comparable specimens from the same locality from 46 to 58$. 

 The ratio does not appear to be affected by age. The breadth 

 varies proportionately more than the length, the breadth being 

 correlated with the breadth of the rostral portion of the skull. 

 Examples of narrow-nosed skulls and broad-nosed skulls from 

 the same locality and of the same sex occur too frequently to 

 render the difference of any importance as a trustworthy charac- 

 ter in comparing small series of skulls from different localities. 

 In two skulls from a single locality in Texas the nasals have a 

 length respectively of 48.8 mm. and 52.9 mm., with a correspond- 

 ing difference in the length of the rostrum, both specimens being 

 middle-aged males. Yet, as will be shown later, the form of the 

 nasals is sufficiently constant to afford a character of much im- 

 portance in the discrimination of subspecies. 



