436 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
is more than twice as wide in some specimens as in others of the same size ; 
the parietal borders are sometimes straight and parallel, but sometimes straight 
and at the same time rapidly convergent; but generally their parietal border 
is more or less concave. ‘Their interorbital breadth is also very variable. The 
lachrymal varies greatly in size, and, while generally triangular, is occasionally 
quadrate. The anterior nasal aperture, while narrower below than above, is 
generally distinctly quadrate, but sometimes decidedly triangular, and often 
more or less approaches a triangular form. It also varies exceedingly in rela- 
tive size. The nasals vary greatly, as already shown, in size and form, inde- 
pendently of the general size of the skull, ranging in length from 1.72 to 
1.90, and in breadth from 0.87 to 1.00. Posteriorly, they may be truncate, 
pointed, or bifurcate. Generally, the greatest width is anterior to the middle, 
the anterior half being abruptly expanded at the middle, or the lateral outline 
may be regularly and moderately swollen. Marked variations are also notice- 
able in respect to the size aud form of the zygomatic processes; in the rela- 
tive size of the malar bone; and in the general details of the lower surface 
of the skull. 
In regard to geographical variation in size, the largest specimens are 
from the Upper Missouri. Of the twenty-six skulls marked in the table as 
“old” or “very old”, six Alaskan ones average 4:91 by 3.61; nine Upper 
Missouri and Platte River skulls average 5.30 by 3.89; three Lake Superior 
and Maine skulls average 4.95 by 3.47; four Rio Grande and Mississippi 
skulls average 5.23 by 3.88. Of the ‘‘middle-aged” series, nine from Arctic 
America average 4.84 by 3.39; five from the Upper Missouri, 4.54 by 3.32; 
a middle-aged Mississippi skull measures 4.70 by 3.35. While the specimens 
are too few to yield positive results, it seems safe to assume that the Upper 
Missouri animal is larger than either the Alaskan or Texan one, and that even 
the Texan one is rather larger than the Arctic American. The southern 
skulls, however, are evidently much more aged than those from the far north, 
so that more specimens, all equally aged, would probably show little, if any, 
difference in size between those from the extreme north and the extreme 
south. While the series of “old” skulls from the Upper Missouri country 
averages much larger than the series from the far north, the average for the 
middle-aged series is reversed, the northern being the larger. This difference, 
however, is more apparent than real; the Upper Missouri series of five 
marked “middle-aged” averaging considerably younger than the more northern 
series of nine similarly marked. 
