89 



Crowns of iutermediate molars truly 

 elliptical. 



Eoot of inferior incisor bat little 

 protuberant on outside of base of 

 condylar ramus; end of mandible 

 thus only 2-prouged, with a knob 

 between. 



Zygomata widest across anteriorly, 

 thence contracting; the width 

 behind little, if any, greater than 

 the intermastoid diameter of the 

 skull. 



Parietals ridged along their line of 

 union with each other. 



Interparietal triangular. 



JSTasals approximately parallel- 

 edged part way, then suddenly" 

 widening. 



Superficies of mastoid bone occu- 

 pying nearly half the occipital 

 surface of the skull on each side. 



Bullse ossese less inflated, quite 

 acute anteriorly. 



Basi-occipital, in the middle, about 

 as broad as the width of the 

 bulla at the same point. 



A pair of broad deep pits on the 

 palate behind, extending forward 

 to opposite the penultimate mo- 

 lars. 



Pterygoids ? (will be found 



differing pppreciably from those 

 of Tlioniomys.) 



Crowns of intermediate molars 

 acute-edged exteriorly. 



Eoot of inferior incisors causing a 

 protuberance on outside of base 

 of condylar ramus nearly as high 

 as condyle itself; end of mandi- 

 ble thus singularly 3-pronged. 



Zygomata regularly convex out- 

 ward, with a sweeping curve, 

 their breadth across posteriorly 

 decidedly greater than the inter- 

 mastoid diameter of the skull. 



Parietals ridged externally, near the 

 squamo-parietal suture. 



Interparietal rather iDcntagonal. 



Nasals widening uniformly from 

 behind forward. 



Superficies of mastoid bone re- 

 stricted to less than a fourth of 

 the occipital surface on each side. 



Bailee ossese more inflated, quite 

 obtuse anteriorly. 



Basi-occipital, in the middle, much 

 narrower than the bulla at the 

 same point. 



A pair of slight pits on the palate 

 behind, not extending beyond 

 the ultimate molars. 



Pterygoids appearing like a bifur- 

 cation into two thin diverging 

 plates of a single median vertical 

 palatal plate. 



In like manner we may proceed to compare some of the principal cra- 

 nial characters of Geomyidce and Saccomyidce. Notwithstanding tlie un- 

 questionable close affinity of these two families, which must stand next 

 to each other in the system, their crania are curiously different in gene- 

 ral appearance and details of contour. The discrepancies are, however, 

 of a superficial character, resulting mainly from the extraordinary mold- 

 ing of the parts in SaccomyidcB. In other words, it is a matter of mere 

 shape, for the most part. There are, however, some curious and more 

 essential features, of which the enormous inflation of various elements 

 of the temporal bone,* and peculiar zygomatic relations posteriorly, 

 are the most remarkable. Probably, going into details, a hundred ac- 

 tual differences between the skulls of Geomyidce and Saccomyidce might 

 be enumerated. I shall content myself with tabulating a few of the 

 more important of these. The comparisons are made between Geomys 

 hursarius and Dipodomys ordi. 



GEOMYIDJE. 



Skull massive, angular, in gene- 

 ral like that of Arvicola, &c. 



SACCOMYIDJE. 



Skull thin and papery, the cor- 

 ners rounded off; the resulting 

 general shape peculiar. 



*" Presenting a ludicrously close resemblance to the buttocks of the squatting human 

 figure," as Baird has aptly said. 



