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498 



MONOGRAPHS OF NORTH AMERICAN Rt)DENTlA. 



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articulation with tlie rostral bones, and with straight sides co'ivergent posteri- 

 orly.* There is a large lachrymal of very irregular shape, extensively scroll- 

 like, very delicate in texture, and loosely attached ; it closes a large aperture 

 lending into the nasal chamber. The orbit is also perforated behind by a 

 single very large foramen of exit of cranial nerves. It is bounded in front, 

 but not roofed over, by the zygomatic plate of the maxillary, not noticeable 

 in character. The extremely delicate malar sutures in front for a long dis- 

 tance, clasp-like, against the zygomatic process; behind, it simply abuts 

 against a slight heel of the squamosal, almost in relation with the tympanic. 

 The singularly displaced "anteorbital" foramen is a large rounded aperture 

 in the side of the snout, communicating directly with the nasal cavity. The 

 nasal bones are parallel-edged for most of their length, but widen a little and 

 become semitubular anteriorly where they project; they are truncate behind, 

 reaching opposite the middle of the jagged fronto-maxillary suture. The 

 sides of the rostrum are contracted below, leaving a very narrow bridge of 

 bone between molars and incisors ; the contracted incisive foramina are 

 bounded behind l)y the maxillaries, though they are chielly pierced in the 

 intermaxillaries. The intermolar portion of the palate is longer than wjde, 

 and a little convergent anteriorly ; the maxillo-palatine suture is opposite the 

 second molar; there is a pair of deep palatal pits opposite the last molars; 

 behind there is a pair of much larger vacuities bounded by palatals in front, 

 sphenoid behind, and pterygoids internally. The latter are simple, straight, 

 nearly parallel processes, bounding the contracted posterior nares, and abut- 

 ting against the petrosals. The orbital plate of the sphenoid is of moderate 

 extent, owing to the size and site of the squamosal. 



The molars in this genus, as in others of the family excepting Dipodomys, 

 are all rooted. They have been said to have four roots, but such is not the 

 case in the specimens I have examined. In P. penicillatus, all the upper 

 molars have three roots apiece, and all the under molars have two roots 

 apiece, excepting the back upper one, which has but one. The front upper 

 molar has one root in advance, corresponding to the anterior lobe of this 

 tooth, and a pair of roots obliquely side by side behind. The next two upper 

 molars have each an exterior pair of roots, lengthwise, and a larger single root 



'Neither this nor any other North American genus shows the ridge of the frontal, which in Htttromft, 

 asflgtired by Peters, makes a boss, or bead, along the margin nf the orbit, and thence extends into the 

 parietal region. This would seem to be a good character of Heteromjiina ; and the akuU of the latter is 

 peculiar iu other respecte — to Judge ih)iu Petera'a flgnre. 



