18/6.] MR. E. R. ALSTON ON THE ORDER GLIRES. 87 



B. BATHYERGiNiE. Palate between the molars not broader than 

 one of the alveoli ; an interpterygoid fissure ; mandible hystricine in 

 form (the angular portion springing from the side of the bony cover- 

 ing of the lower incisor). Ethiopian. Recent genera : — 



4. Bathyergus, Illiger, Prod. Syst. Mamm, p. 8G ... (1811). 



Eyes small ; no ear-conch ; fore feet with very long claws ; hind feet 

 with the third digit longest ; tail short, hairy. Skull massive, occipital 

 plane perpendicular ; nasals very narrow ; parietals very small ; infra- 

 orbital foramen very small, subcircular ; angle of mandible pointed. 

 Upper incisors enormously large, broad, grooved. One premolar above 

 and below. Molars with indenting enamel-folds in youth only. 



5. Georychiis, lUiger, Prod. Syst. Mamm. p. 87 .... (1811). 

 Externally like Bathyergus, but the claws, especially of the fore 



feet, shorter and weaker, skull with the profile more arched, the 

 occipital plane slightly sloped forward, and the angle of the mandible 

 rounded. Upper incisors long, arched forwards, plain. Grinding- 

 teetli as in Bathyergus. 



6. Heliophohius, Peters, Monatsb. Ak. Berlin, 1846, p. 239 



(1846). 



DiiFers externally from Georychus in the second digit of the hind 

 feet being the longest. Skull with the infraorbital opening smaller, 

 the bony palate shorter, and the coronoid process larger. Incisors 

 plain. Three premolars above and below ; but sometimes two only are 

 developed. 



Family V. Geom'xid^. 



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

 Malar extending forward to the lachrymal. Squamosals extremely 

 large. Angular portion of mandible strongly twisted. Large 

 cheek-pouches, opening on the cheeks outside the mouth. Upper 

 lip not cleft. Other external characters very variable. Nearctic 

 and Neotropical. Two subfamilies : — 



A. Geomyin^e. Incisors broad. Skull massive; infraorbital 

 opening very small ; mastoid not appearing on the top of the skull ; 

 malar stout. Form thickset ; eyes small ; ear- conch rudimentary ; 

 limbs short, subequal ; fore claws very large. Recent genera : — 



1. C?eomys, Rafinesque, Am. Monthly Mag. ii. p. 4.5 . . (1817). 

 Fore feet large, with very large compressed curved claws ; tip of 



tail naked ; cheek-pouches large. Skull very massive ; zygomatic 

 arches stout. Upper incisors deeply grooved. Crowns of premolars^ 

 divided into two subequal lobes. 



2. Thomomys, Max. Prinz zu Wied, Nov. Act. Ac. Car. Leop. 

 xix. i. p. 383 (1839). 



Fore feet comparatively small, with moderate claws ; cheek-pouches 

 moderate. Skull less massive ; zygoma more slender. Upper inci- 

 sors plain, or with merely a fine groove near their inner edge. An- 



