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232 



MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



25 to 100 — one-fourth. Tlie 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 n^tch is a deep emargination on the 

 median line posteriorly carried to o|)posite the space between the last and 

 penultimate molars, and on either side of this emargination lies a conspicu- 

 ous dcej) ovate fos^a; there is the same step down to the level of the margins 

 of these fbssoe tliat is seen in Arvicola proper, but the fossae are larger and 

 deeper. (In Myodes, these fossae really do occur, and of great size ; but 

 they are so mucii hidden, by being tucked under the apparent termination 

 of the palate, that the palate seems to have essentially the same construction 

 as in Evototnys; i. e., to end as a broad shelf, nearly transverse.) 



In general configuration, the skull is near that of Myodes, The zygo- 

 matic widtii equals or just exceeds lialf the total length. The interorbital 

 constriction is very great, the skull being here narrower or at most no broader 

 than tlie 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- 

 rics are of about equal length, neither reaching the interorbital constriction. 

 Tlio zygomatic arches are vvidespread, 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 ArvkoUna. The anteorbital foramina open entirely underneath the 

 anterior roois of the zygoma, so that the latter scarcely show from above tlie 

 slight nick usually evident in other genera; the foramina themselves havo 

 the ordinary pyriform shape. 



The rostral part of llie 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 beliind a perpendicular line let fall against tiie faces of the 

 incisors. Tiie sphenoidal ate are l)road and short, flaring away from tiie 

 rather small and short pterygoids. Tiie basioccipital and basispiienoid are 

 narrow, so that the tynqmnic bulla;, which arc large, lie with sligiit mutual 



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