22 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
25 to 100—one-fourth. The series of opposite sides are strongly convergent 
anteriorly, so that they would about meet if produced little beyond the incisors. 
This is nearly as in Myodes, where, however, the convergence is rather greater. 
Singularly enough, the formation of the palate behind does not agree with either 
Myodes or Evotomys, but is nearly as in Arvicola proper—amphibius or ripa- 
rius, for instance. The post-palatal notch is a deep emargination on the 
median line posteriorly carried to opposite the space between the last and 
penultimate molars, and on either side of this emargination lies a conspicu- 
ous deep ovate fossa; there is the same step down to the level of the margins 
of these fossee that is seen in Arvicola proper, but the fosse are larger and 
deeper. (In Myodes, these fosse really do occur, and of great size; but 
they are so much hidden, by being tucked under the apparent termination 
of the palate, that the palate seems to have essentially the same construction 
as in Lvotomys; 7. e., to end as a broad shelf, nearly transverse.) 
In general configuration, the skull is near that of Myodes. The zygo- 
matic width equals or just exceeds half the total length. The interorbital 
constriction is very great, the skull being here narrower or at most no broader 
than the rostral part. The cranium protrudes far into the orbital cavities, 
with a prominent angular outline. The incisive foramina are short and nar- 
row, opening considerably behind the incisors, and closing again as much in 
front of the molars. The nasal bones and nasal branches of the premaxilla- 
ries are of about equal length, neither reaching the interorbital constriction. 
The zygomatic arches are widespread, and have the peculiar perpendicular 
expansion of the jugal bone and of the malar spur of the maxillary, form- 
ing quite a broad oblique plate, as in Myodes. The difference is very notice- 
able on comparison with the much slenderer and more terete zygomata of 
other Arvicoling. The anteorbital foramina open entirely underneath the 
anterior roots of the zygoma, so that the latter scarcely show from above the 
slight nick usually evident in other genera; the foramina themselves have 
the ordinary pyriform shape. 
The rostral part of the skull is remarkably short, thick, and blunt. Its 
length, from anterior root of zygomata to tip of nasal bones, scarcely exceeds 
its width, and is less than its depth. The nasal bones are short, broad, and 
obtuse, ending behind a perpendicular line let fall against the faces of the 
incisors. The sphenoidal ale are broad and short, flaring away from the 
rather small and short pterygoids. The basioccipital and basisphenoid are 
narrow, so that the tympanic bulle, which are large, lie with slight mutual 
