SACCOMYLLVE— SUBFAMILY PEKOUNATUIDIN.E. 495 



a third subfamily. In such case, some few of the characters of the three 

 would be as follows : — 



Dipodomyin/E. — Molars rootless; upper incisors compressed, sulcate 

 Temporal region enormously inflated. Pelage comparatively soft. 



PEROGNATHiDiNiE. — Molars rooted ; upper incisors compressed, sulcate. 

 Temporal region moderately inflated. Pelage comparatively hispid. 



Heteromyin^e. — Molars rooted; upper incisors broad, smooth. Tem- 

 poral region "not inflated". Pelage "mixed with flattened spines". 



Subfamily PE HOGNATHIDTM!. 



c= Perognathits (genus), of Authohs. 



= Perogtiathidinas, Coues, Proc. Pbila. Acad. 1675, 278. 



The characters of the group having been already sufficiently elucidated, 

 it'remains to note the two genera by which it is represented in North Amer- 



357-359) a more satisfactory account of the characters of the genus than I have seen elsewhere. As this 

 account furnishes important information pertinent to the general subject, I transcribe his diagnosis of 

 the genus, and give a version of those portions of the description of the species which tend to further 

 elucidate the generic characters. 



" Deut.es iucisivi pagiua antica laevigati exserti, molares ^ complicati radicati ; labrum integrum; 

 rictus perparvus ; rostrum prominens, rbinario nudo; sacculi bucculesexterni pilosi ; auriculae mediocres; 

 vellns setosum, setis lanceolatis, canaliculars ; pedes pentadactyli ; cauda annulata, brevipilosa. Os 

 interparietale latum, tempora non inflata. 



" Habitus of Mus. Thumb prominent, with rounded nail, shorter than the other digits, which run 

 5th, 2d, 4th, and 3d, the latter longest. Relative leugths of the digits of the hind foot as in Isomys ; 5th 

 but little longer than 1st, 3d longest ; claws rather longer than those of the fore foot, that of the 2d toe 

 longest, convex on the iuuer side, flat ou the outer. Soles naked, with the usual tubercles. Incisors 

 compressed, the horizontal cross-section triangular, with rounded hinder angle, and apparently broader 

 than long. Molar series parallel ; last molar in each jaw smallest ; 2d and 3d upper molars of equal size 

 and notably smaller thai the 1st ; anterior molar of lower jaw but little larger than the next two .... 



" Incisors yellow, the lower paler colored than the upper. Anterior superior molar with three 

 separate enamel-tubes— an anterior, louger and narrower, a posterior shorter and broader, aud a third 

 much smaller oval one on the inner side of the tooth in a recess where the other two stand apart ; 

 second upper molar with a deep internal and slight external emargiuation, or fold of enamel ; third with 

 a slight exterior emargination, whicb is either continuous with a transversely-elliptical interior island 

 of enamel, thus forming a deep enamel-fold, or else only just reaches this island without fusing with it ; 

 back upper molar with the enamel-pattern like that of the second one, but the posterior half of the 

 tooth considerably smaller than the anterior portion. 



"Anterior inferior m^lar with two enamel-tubes, the anterior of which, somewhat smaller than 

 the other, is emarginate before and behind ; the three following teeth with a deep inner and slight outer 

 emargination, as on the upper teeth, which are either separated or fused together, in which latter case 

 two separate enamel-tubes result. 



"The rostral portion of the skull is narrow ; the nasal bones and intermaxillaries project beyond 

 the incisors as in allied genera, and the intermaxillaries form anteriorly a sharp perpendicular ridge. 

 The structure of the small incisive foramina, the shape of the lachrymal bones aud infraorbital foramina 

 are quite as in Geomys. The latter are wholly in the superior maxillary and are separate from the 

 nasal cavity. Since, however, the bony septum is in this case extremely thin , and easily mutilated, the 

 deceptive appearance of communication between the infraorbital foramina and the nasal passages may 

 be accidentally produced. The frontal bone, as in Mux, forms a sharp supraorbital ridge, which continues 

 on to the parietal, thus forming with its fellow of the opposite side the boundary of a level area, with 

 which the interparietal, twice as broad as long, is posteriorly continuous, whilst the temporal fossa is 

 excavated [a strong character of Heteromyinu: — there is no such formation in Perognath.diwas. — Tit.], lho 

 lower jaw strongly resembles that of Perognathus in the form of the coronoid process and mandibular 

 angle; the deep pit which is found in Geomys to the outer side of the posterior molar is here wanting." 



