18/6.] MR. E. R. ALSTON ON THE ORDER GLIRES. 89 



3. Issidioromys (Croizet), De Blainville, C. R. Ac. Paris, x. p. 932 



(1840). 



Grinding-teeth rootless, with open reentei-ing folds dividing their 

 crowns into heart-shaped lobes ; the subsidiary folds only repre- 

 sented here and there by a minute isolated enamel-loop. 



Family VII. Dipodid^e. 

 Incisors compressed. Premolars present or absent. Grinding-teeth 

 rooted or rootless, not tuberculate, with more or fewer transverse ena- 

 mel-folds. Skull with the brain-case short and broad ; infraorbital 

 opening rounded, very large (often as lai'ge as the orbit) ; zygomatic 

 arch slender, curved downwards ; the malar ascending in front to 

 the lachrymal in a flattened perpendicular plate ; facial surface of 

 maxillaries minutely perforated ; mastoid portion of auditory bullae 

 usually greatly developed. Metatarsal bones greatly elongated, often 

 fused into a cannon bone. Form gracile ; front portion of body and 

 fore limbs very small ; hind limbs long and strong, with from three 

 to five digits; tail long, hairy. Three subfamilies: — 



A. Jaculin.e*. One premolar above. Grinding-teeth rooted. 

 Cervical vertebrae free, metatarsals separate. Hind feet with five 

 developed digits. Tail sparsely haired. Nearctic. Recent genus : — 



1. Jaculus, Wagner, Syst. Amph. &c. p. 23 (1830). 



(Characters those of the subfamily). 



B. Dipodin^e. Premolars present or absent. Grinding-teeth 

 rooted. Cervical vertebrae more or less ankylosed. Metatarsals 

 united in a cannon-bone. Hind feet with only three digits function- 

 ally developed. Tail thickly haired, often tufted. Palaearctic and 

 Ethiopian. Recent genera : — 



2. Bipus, Gmelin, Syst. Nat. ed. 13, i. p. 157 (1/88). 



Hind feet with three digits only ; tail cylindrical, tufted. Skull with 



occipital region very broad, auditory bullae enormously developed, 

 infraorbital opening with a separate canal for the nerve. Incisors 

 grooved. Premolars absent or almost rudimentary, and found above 

 only. 



3. Alactaga, F. Cuvier, P. Z. S. 1836, p. 141 (J83G). 



Hind feet with five digits, of which the first and fifth do not 



reach the ground ; tail cylindrical, tufted. Skull with the occipital 

 region less broad, auditory bullae smaller, infraorbital opening with 

 no separate canal for the nerve. Incisors plain. One very small pre- 

 molar present above only. 



4. Platycercomys, Brandt, Bull. Ac. St. Petersb. p. 209 . (1844). 

 As in Alactaga, but the hind limbs proportionally shorter, and 



* Since the above went to press, Dr. E. Coucs has published a paper in which 

 he rejects the generic names Jaculus and Mcrioncs as preoccupied, substitutes 

 Zajri's, and regards the form as the tvpe of a distinct family, Zapodida (Eull. 

 "U.S. Geol. Surr. v. pp. 253-202). 



