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Show 78 MR. E. R. ALSTON ON THE ORDER GLIRES. [Jan. 18, Family III. ISCHYROMYID^E (fam. nov.)*. Dentition as in Sciurida. Skull resembling Castorida, but with the infraorbital opening large, a sagittal crest, no postorbital processes, palate broad, basioccipital keeled. Miocene of North America. Fossil genus :- Ischyromys, Leidy, Proc. Acad. Philad. 1856, p. 89 . . (1856). (Characters those of the family.) Family IV. HAPLODONTID^E. T W O premolars above and one below, the first upper premolar small; grinding-teeth rootless, simple, and prismatic. Skull much depressed, no postorbital processes, infraorbital opening small, angular portion of mandible much twisted. Tail short, cylindrical, hairy. Nearctic. Recent genus:- Haplodon = Aplodontia, Richardson, Zool. Journ. iv. p. 334 (1829). (Characters those of the family.) Family V. CASTORIDEE. One premolar above and below; grinding-teeth subequal, semi-rooted or rootless, with reentering enamel-folds. Skull massive, no postorbital processes, infraorbital opening small and placed low, a sagittal crest, angle of mandible rounded. Carpus with a large accessory ossicle. Stomach with a glandular appendage, excretory and generative organs opening into a common cloaca. Tail broad, flattened, spatulate and reticulated. Hind feet fully webbed. Palaearctic and Nearctic. Recent genus :- 1. Castor, Linnaeus, Syst. Nat. i. p. 78 (1766). (External characters those of the family.) Upper grinding-teeth subequal, each with one internal and three external enamel-folds; the lower similar but reversed; the subsidiary folds not soon isolated from the exterior. Parietals narrow, parallelogrammatic; interparietal triangular ; basioccipital concave. Fossil genera:- 2. Diobroticus, Pomel, Arch. Bibl. Univ. Geneve, ix. p. 167*t*. (1848). Skull much as in Castor. Third upper molar and lower premolar elongate, with four enamel folds, the rest with only two ; all the folds soon isolated. 3. Stenofiber, Is. Geoffroy, Revue Encyclopedique. . . . (1833). Parietals not parallelogrammatic ; interparietal subhexagonal; basioccipital not concave. Grinding-teeth as in Castor, the subsidiary folds sooner isolated. * It seems probable that Pseudotomys, Cope, Proc. A m . Phil. Soc. 1872, p. 467, from Eocene of North America, may prove to belong to this family. t = Trogontherium Owen (nee Fischer), Brit. Foss. M a m m . p. 184; Geol. Mag. vi. pp. 49-56 (cf. Gervais, Zool. et Paleont. Generates, pp. 80-84). |