NATURAL SCIENCES OF PHILADELPHIA. 281 



ture leading into the nasal chamber. The orbit is also perforate 

 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 distance 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 semi-tubular anteriorlj' 

 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 by the maxillaries, though they 

 are chiefly pierced in the intermaxillaries. The intermolar por- 

 tion of the palate is longer than wide, and a little convergent 

 anteriorly ; the maxillo-palatine suture is opposite the second mo- 

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

 behind there is a pair of much larger vacuities bounded b} r palatals 

 in front, sphenoid behind, and pterygoids internally. The latter 

 are simple, straight, nearlj- parallel processes, bounding the con- 

 tracted posterior nares, and abutting 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 on the interior side; the back upper molar is 

 simply single-rooted. The under molars have each a pair of roots, 

 alligned lengthwise in a single series, but the two roots of the 

 back lower molar are imperfectly distinguished. With these last 

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