470 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



only a notch and nol a foramen results, the correspondence of the whole 

 opening with the pyriform slit of the Murida is very evident, and the relation 

 of the parts is fully established, though the shape is quite different. 



The contour of the parts surrounding the foramen is such thai the zygo- 

 matic process of the maxillary stands out at a righl angle at a point scarcely 

 above the level of the alveoli. The anterior root of the zygoma is hence 

 notably depressed in position; there being no forward-upward reach of the 

 lower border of this arch, so evident in Murida. The zygoma, in fact, is 

 nearly horizontal in all of its length along the under side ; but anteriorly the 

 upper edge rises prominently, in consequence of the unusual extension of the 

 malar up the maxillary, already mentioned. The malar runs all the way up 

 to the lachrymal bone, affording a circumstance which I believe to be rare, 

 namely, a lachrymo-malar suture like that which is found in Dipodida and 

 PedeAidcp. This ascending spur of the malar is, moreover, expanded into a 

 rather broad lamina, partly defending the orbit, thus supplying a wall that, in 

 most cases, is afforded by expansion of the zygomatic process of the maxil- 

 lary ; the latter being in this case of styloid character. In its continuity, 

 the malar is a slender rod ; behind, it underlaps a short spur of the squamosal 

 with simple squamous suture. 



The general shape of the orbit is much the same as in Mus. In both, 

 the squamosal forms much of the posterior orbital wall; the orbito-sphenoid 

 being correspondingly reduced. The antero-exterior corner of the parietal 

 reaches to the brim of the orbit. 



The rostral portion of the skull bears to the rest about the same propor- 

 tion as in Mus, and is equally attenuate anteriorly, though thicker at the base, 

 and consequently more tapering. The ends of the nasals project conspicu- 

 ously beyond the plane of the incisors; behind, these bones terminate oppo- 

 site the ends of the intermaxillaries ; the suture of the frontal with each of 

 them, as well as with the maxillaries, being nearly in one transverse jagged 

 line. The intermaxillaries develope a strong alveolar plate, separating the 

 superior incisors for nearly half their length ; this, with the projection of the 

 nasals and backward set of the much-curved teeth, results in a snout strikingly 

 like that of the Saccojnyidcv. The feeble, retreating under jaw, densely hairy 

 upper lip, and small nasal pads bear out this resemblance in the external 

 physiognomy. 



As in Mus, the inlet-orbital constriction is moderate, being about as wide 



