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Show 1876.] MR. E. R. ALSTON ON THE ORDER GLIRES. 89 3. Issidioromys (Croizet), De Blainville, C. R. Ac. Paris, x. p. Grinding-teeth rootless, with open reentering folds dividing their crowns into heart-shaped lobes; the subsidiary folds only represented here and there by a minute isolated enamel-loop. Family VII. DIPODIDA. Incisors compressed. Premolars present or absent. Grinding-teeth rooted or rootless, not tuberculate, with more or fewer transverse enamel- folds. Skull with the brain-case short and broad ; infraorbital opening rounded, very large (often as large 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. DIPODINJE. 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 functionally developed. Tail thickly haired, often tufted. Palaearctic and Ethiopian. Recent genera :- 2. Dipus, Gmelin, Syst. Nat. ed. 13, i. p. 157 (1788). 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 (J836). 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 premolar present above only. 4. Platycercomys, Brandt, Bull. Ac. St. Pe'tersb. p. 209 . (1844). As in Alactaga, but the hind limbs proportionally shorter, and * Since the above went to press, Dr. E. Coues has published a paper in which he rejects the generic names Jaculus and Meriones as preoccupied, substitutes Zapus, and regards the form as the type of a distinct family, Zapodidce (Bull U.S. Geol. Surv. v. pp. 253-262). |
