43(3 MONOGRAPHS OK NORTH AMERICAN KODENTIA. 



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 mora 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 and 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 considerablyyounger than the more northern 

 series of nine similarly marked. 



