4G4 MONOGKAPnS OF NORTH AMERICAN EODENTIA. 



The Zapodidcc may be considered a connecting link (through GrrbUlina ?-) 

 between the "central" or generalized structure which characterizes the 

 MuridcB, and the "peripheral" or specialized type of structure which culmi- 

 nates in Dipodida. The family may be established upon the following com- 

 bination of characters, superimposed upon those that belong to the Myomorph 

 series at large.* 



Chars.— Teeth, 18: I. \E$; C.^f; Pm. ^; M.|^|. Superior incisors 

 compressed, sulcate ; premolar small ; molars rooted. Cervical vertebrae 

 unanchylosed (cf. Dipodidce). Cranial portion of the skull shorter and broader 

 than in Muridce. Auditory bullae transverse (cf. Muridcz), without special 

 development (cf. Dipodidce). Anteorbital foramen large, rounded (cf. typical 

 Muridce), with a supplementary nick, or additional foramen, at its lower por- 

 tion. Malar mounting the zygomatic process of the maxillary to effect suture 

 with the lachrymal (cf. MurideB). Zygomatic arch styloid, much depressed. 



* I have but recently received Mr. E. R. Alston's convenient and valuable paper on the classification 

 of the Rodents (P. Z. S Jan. 1876, pp. 61-98, pi. iv), too late for such use as I should have wished to make of 

 it in the preparatiou of my monograph of the Muridw, which, though still (Dec. 1876) unpublished, has 

 been for some months in the metal. 



In this memoir, the author arranges Zapus (under the name of Jaculus) as one of three subfamilies 

 of Dipodidw, which constitute the seventh and last of his series of Myomorphs, Muridce being ranged 

 third. In order to fully exhibit the grounds upon which he differs with me respecting the position of 

 Zapus, — though our difference is chiefly in our respective estimates of comparative degrees of inter-rela- 

 tionship and the taxonomic value of mutually conceded characters, — his diagnoses of Murida; and of 

 Dipodidce, and its three assigned subfamilies, are subjoined. 



" Family 111. Muridce. Lower incisors compressed ; no premolars (except in Sminthince) ; molars 

 rooted or rootless, tuberculate or with angular enamel-folds. Frontals contracted. Infraorbital opening 

 iu typical forms high, perpendicular, wide above and narrowed below, with the lower root of the maxillary 

 zygomatic process more or less flattened into a perpendicular plate ; very rarely the opening is either 

 largo and oval or small and subtriaugular. Malar short and slender, generally reduced to a spliut 

 between the maxillary and squamosal processes. External characters very variable. Pollex rudimentary, 

 but often with a small nail. Tail generally subuaked and scaly, rarely densely haired. Cosmopolitan. 

 Ten subfamilies. * * *." 



*' Family VII. Dipodidce. Incisors compressed. Premolars present or absent. Griuding-teeth 

 rooted or rootless, not tuberculate, with more or fewer transverse euamej-folds. Skull with the braiu- 

 case short and broad ; infraorbital opening rounded, very large (o'ften as large as the orbit) ; zygomatic 

 arch slender, curved downward ; the malar ascending iu front to the lachrymal iu a flattened perpen- 

 dicular plate ; facial surface of the maxillaries minutely perforated ; mastoid portion of auditory bulla} 

 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. Jaculinw. One premolar above. Griuding-teeth rooted. Cervical vertebra) free, metatarsals 

 separate. Hind feet with five developed digits. Tail sparsely haired. Nearctic. [Oue genus, — Jaculus.'] 



" B. Vipodina;. Premolars present or absent. Grinding-teeth rooted. Cervical vertebras more or 

 less ankylosed. Metatarsals united iu a cannon-bone. Hind feet with only three digits functionally 

 developed. Tail thickly haired, often tufted. Palasarclic and Ethiopian. [Three genera, — Dipus, Alactaga, 

 I'latycercomys. ] 



" C Pedvtinw. Oue premolar above and below. Grinding-teeth rootless. Cervical vertebra free, 

 Metatarsals separate. Hind feet with four well-developed digits, with short broad hoof-like nails. Tail 

 bushy throughout, not tufted. Ethiopian." [One genus,— i'id< /to. 1 



