88 MR. E. R. ALSTON ON THE ORDER GLIRES. [Jan. 18, 



terior lobes of premolars small. A third upper molar small, sub- 

 circular. 



B. Heteromyin^e. Incisors narrow. Skull more delicate, with 

 the mastoid appearing largely on its top ; infraorbital opening not 

 defined, through the non-development of the lower root of the maxil- 

 lary zygomatic process ; malar slender. Cervical vertebrse sometimes 

 ankylosed (as in Dipodinee). Form slender ; hind limbs and tail 

 elongated ; eye and ear moderate or large. Recent genera : — 



3. Dipodomys, Gray, Ann. & Mag. Nat. Hist. vii. p. 521 (1840). 



Ear large, rounded ; tail long, densely haired, tufted ; soles hairy ; 

 pollcx very small, with a small claw. Cheek-pouches large. Skull 

 with nasals produced in front ; upper part of zygoma produced into 

 a flat plate articulating with the frontals ; interparietal very small, 

 narrow ; auditory bullse enormous, projecting far beyond the occi- 

 pital plane. Incisors plain. Grinding-teeth rootless, at first with 

 slight indenting enamel- folds, afterwards simple. 



4. Perognathus, Max. Prinz zu Wied, Nov. Act. Ac. Car. Leop. 

 xix. i. p. 369 (1839). 



Ears shorter ; tail thinly haired ; soles more or less naked ; pollex 

 with a flat nail. Skull less modified ; nasals not so much produced ; 

 zygoma not developed into a flat plate ; interparietal broad ; auditory 

 bullse not projecting behind the occipital plane. Incisors grooved. 

 Grinding-teeth rooted, tuberculate in youth, afterwards with isolated 

 enamel-loops. 



5. Heteromys, Desmarest, Mammalogie, p. 313 (1820). 



Like Perof/nafhtts, but the fur bristly, mixed with flattened spines ; 

 tail shorter, clad with large scales and scattered hairs. Skull with 

 sharp supraorbital ridges; interparietal very broad. Upper incisors 

 plain. Grinding-teeth as in Perognathus. 



Family VI. Theridomyid.e. 



One premolar present above and below. Grinding-teeth rooted or 

 rootless, not tuberculate, with more or fewer transverse enamel-folds. 

 Infraorbital opening large, suboval. Palate somewhat contracted in 

 frout and emarginate behind. Eocene and Miocene of Europe. Fossil 

 genera : — 



1 . Theridomys, Jourdan, Compt. Rend. Ac. Paris, v. p. 483 (1837). 

 Grinding-teeth rooted, with three or four reentering enamel-folds, 



which become isolated enamel-loops in the worn teeth. 



2. Archceomys, De Laizer et De Parieu, C. R. Ac. Paris, viii. p. 206 



(1839). 

 Grinding-teeth rootless, the enamel-folds continued diagonally 

 across the crowns, which are thus divided into laminae, of which the 

 anterior is the largest above, while they are subcqual below. 



