78 MR. i;. R. ALSTON ON TH IT ORDER GLIRES. [Jan. 18, 



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



Dentition as in Scinridce. Skull resembling CastoridcB, 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 . . (18.')6). 

 (Characters those of the family.) 



Family IV. HaplodontidtE. 



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. Castorid.e. 

 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, Palsearctic 

 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. 167t. 



(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, Geoifroy, Revue Encyclojiedique. . . . (1833). 



Parietals not parallelogrammatic ; interparietal subhexagonal ; basi- 

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

 folds sooner isolated. 



* It seems probable that Pseudotomys, Cope, Proc. Am. Phil. See. 1872, 

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



t =Trogon^hrrunn Owen (uec Fischer), Brit. Foss. Maram. p. 184; Geol. 

 Mag. vi. pp. 49-.'it) (cf. Gervais, Zool. et Paleont. G^nerales, pp. 80-84). 



