90 



Interorbital space the narrowest 

 part of the skull — narrower than 

 rostrum. 



Occipital region approaching a 

 plane surface, without median 

 emarginatiou. 



Kasal bones not iDroduced beyond 

 vertical plane of incisors ; ros- 

 trum broad, parallel-sided. 



Parietals small, linear, remote from 

 the orbits. 



Occipital of an ordinary character, 

 not attaining top of skull. 



Temporal bone, though of great ex- 

 tent, not remarkably inflated. 



Squamosal roofing most of the cere- 

 bral cavity. 



Zj'gomata of an ordinary character, 

 with the usual counectious. 



Tympanic, a contracted tube. 

 Petrosals discrete from each other, 

 in contact with basioccipital. 



Mastoid excluded from roof of 

 cerebral cavity. 



Zygomatic process of maxillary, 

 a plate with merely thickened 

 upper border. 



Palatal outline strongly ascending 

 and arched anteriorly ; molars 

 far below level of zygomata. 



Incisors large, parallel-edged, 

 scarcely conv^erging. 



Anterior molar, a double i:)rism. 



Koot of inferior incisor protube- 

 rant posteriorly. 



Large erect falcate coronoid, over- 

 topping condyle. 



&c. &G. 



Interorbital space expanded, very 

 much broader than the rostrum. 



Occipital region formed chiefly of 

 enormous bulging mastoids, with 

 deep median emargiuation. 



Nasal bones produced far beyond 

 incisors; rostrum attenuated, ta- 

 pering. 



Parietals large right-angled tri- 

 angles, together as broad as the 

 frontal, reaching orbits. 



Occipital reduced and of peculiar 

 shape ; a part of it mounting the 

 top of the skull. 



Temporal bone unique in its enor- 

 mous size and inflation, being 

 blown up like a bladder; the 

 swollen mastoids forming most 

 of the occipital plane ; the two 

 temporals larger than all the rest 

 of the skull together,* 



Squamosal restricted to the orbit. 



Zygomata thread-like in most of 

 their extent, and greatly de- 

 pressed in position ; the malar 

 hone abutting against the tym- 

 panic. 



Tympanic, an inflated vestibule. 



Petrosals in mutual contact at their 

 extremities, and fissured away 

 from basi-occipital. 



Mastoid roofing most of cerebral 

 cavity. 



Zygomatic process developing into 

 a shield over much of the orbital 

 space. 



Palatal profile nearly straight and 

 horizontal ; molars on a level 

 with the zygomata. 



Incisors small, acuminate, con- 

 vergent. 



Anterior molar, a single prism. 



Koot of interior incisor not promi- 

 nent posteriorly. 



Minute prickle-like sloping coro- 

 noid, far below level of condyle. 



&c. &c. 



* If the sense of hearing of Dipodomys be coordinated with the osseous development of the 

 auditory apparatus, it must be extraordinarily acute. 



