78 MR. E. R. ALSTON ON THE ORDER GLIRES. [Jail. 18, 



Family III. Ischyromyid.e (fam. nov.)*. 



Dentition as in Sciuridce. Skull resembling Castoridce, but with 

 the infraorbital opening large, a sagittal crest, no postorbital 

 processes, palate broad, basioceipital keeled. Miocene of Nortli 

 America. Fossil genus : — 



Ischyromys, Leidy, Proc. Acad. Philad. 1856, p. 89 . . (1856). 

 (Characters those of the family.) 



Family IV. Haplodontid^e. 



Two 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. Castor idm. 

 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 acces- 

 sory ossicle. Stomach with a glandular appendage, excretory and 

 generative organs opening into a common cloaca. Tail broad, flat- 

 tened, spatulate and reticulated. Hind feet fully webbed. Paleearctic 

 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 ; basioceipital concave. 

 Fossil genera : — 



2. Diobroticus, Pomel, Arch. Bibl. Univ. Geneve, ix. p. 167 f. 



(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. Stenojiber, Is. GeofFroy, Revue Eucyclope'dique. . . . (1833). 



Parietals not parallelogrammatic ; interparietal subhexagonal ; basi- 

 oceipital not concave. Grinding-teeth as in Castor, the subsidiary 

 folds sooner isolated. 



* It seems probable that Pseudotomys, Cope, Proe. Am. Phil. Soc. 1672, 

 p. 467, from Eocene of North America, may prove to belong to this family. 



t =-Trogonfherium Owen (nee Fischer"), Brit. Foss. Mamm. p. 1H4 ; Qeol. 

 Mag. vi. pp. 49 56 (of. Q-ervais, Zool. et Paleont. Generates, pp. HO M | 



