RQDENTIA-ARVICOLINAE-—ARVICOLA. 
511 
posterior third. Tail as long or longer than tho head ; the hairs on the posterior third longer 
than those anteriorly. 
In the American specimens hitherto examined, the tail is twice as long as the hind foot, less 
than one-third the head and body, darkest at the tip. 
The skull of the Arvicola possesses certain very uniform cranial characteristics, although 
variations in subordinate points afford the means of establishing subdivisions which greatly 
facilitate the comprehension of the species. It will not do, however, to attach too much 
importance to all the variations of bones and teeth, as in no group do we find a greater latitude 
within specific limits. It is only by the examination of many specimens that we can seize an 
average of conditions that may he considered as typical. 
The principal points in which the subdivisions of Arvicola differ, are in the extent of the 
nasal branch of the intermaxillary hone, the nature of the bony palate, the number of closed 
triangles in the molar teeth, with the position and number of the salient angles, and some other 
features hereafter to he mentioned. These usually vary little in different species of one group. 
The specific variations are in the proportions of the skull, the shape of the occipital foramen, 
the size of the teeth, and rarely in the outline of the enamel folds. I have not been able to 
appreciate any constant difference in the sexes, hut the adults differ from the young in the size 
and proportions of the skull, the greater angularity of outline, &c. 
As a whole, the skulls of American Arvicolae differ from the European, as in only one species 
{Arvicola agrestic from Sweden) have I found an accordance in every general respect with an 
American. In the above mentioned Arvicola the skull is almost entirely like that of A. riparia, 
although certain specific differences may ho noted. An exception must, however, he made for 
the species of Hypadaeus, in which there is a close concordance. 
After an examination of many skulls of Arvicolae, European and American, (over 20 species,) 
I am inclined fully to agree with Keyserling and Blasius, in separating Hypadaeus from 
Arvicola , and even to raise it to generic rank. It is true the differences are not very great, hut 
they are strongly marked and constant; and in a group where the number of closely allied 
species is so large, what would otherwise he a trifling character may take an important place. 
I find also some characters to distinguish the two, to which the above mentioned authors do not 
allude, and which are even more striking than any they mention. The chief external feature of 
Hypadaeus is in the large ears prominent above the fur, the surfaces densely coated with short, 
close hair, as in the squirrels. The skull differs in the palatine vault, the double roots to the 
molars, the shape of the crown, and some other features. 
The skull of the restricted genus Arvicola is short and broad, as well as high. The length is 
a little more than twice the greatest width, which occurs at about the middle of the axial line. 
Viewed from above, the muzzle is narrow and its outlines nearly parallel; it occupies the 
anterior third or fourth of the head, which suddenly widens along the anterior wall of the 
zygomatic arch, and then curves round to the temporal region. The confluent orbital and 
temporal fossae are large, their posterior internal boundary generally sharply angular in the 
adult. The nasal hones are about one-fourth'the length of the head ; the nasal branch of the 
intermaxillary is as long or a little longer than the nasal hones, and reaches to a point between 
the roots of the zygomatic arch or a little beyond, as far as the small lachrymal bone, or to the 
inter-orbital region. The occipital foramen is sometimes higher than wide, and suh-triangular, 
or as high as wide, and rounded. The auditory bullae are large. The bony palate is nearly 
plane between the two anterior molars of each side ; opposite the last molar on either side it is 
