464 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
The Zapodide may be considered a connecting link (through Gerbillina ?) 
between the “central” or generalized structure which characterizes the 
Muride, and the “peripheral” or specialized type of structure which culmi- 
nates in Dipodide. The family may be established upon the following com- 
bination of characters, superimposed upon those that belong to the Myomorph 
series at large.* 
Cuars.—Teeth, 18: L24; C.9=2; Pm. 2; M.==3. Superior incisors 
compressed, sulcate; premolar small; molars rooted. Cervical vertebrae 
unanchylosed (cf. Dipodide). Cranial portion of the skull shorter and broader 
than in Muride. Auditory bulle transverse (cf. Muride), without special 
development (cf. Dipodide). Anteorbital foramen large, rounded (cf. typical 
Muride), 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. Muride). Zygomatic arch styloid, much depressed. 
*T have but recently received Mr. E. R. Alston’s convenient and valuable paper on the classification 
of the Rodents (P. Z. 8S. Jan. 1876, pp. 61-98, pl. iv), too late for such use as I should have wished to make of 
it in the preparation of my monograph of the Muride, which, though still (Dec. 1876) unpublished, has 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 Dipodida, which constitute the seventh and last of his series of Myomorphs, Muride 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 Muride, and of 
Dipodida, and its three assigned subfamilies, are subjoined. 
“ Family 11. Muripa. Lower incisors compressed ; no premolars (except in Sminthine) ; molars 
rooted or rootless, tuberculate or with angular enamel-folds. Frontals contracted, Infraorbital opening 
in 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 
large and oval or small and subtriangular. Malar short and slender, generally reduced to a splint 
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. * * von 
“ Family VII. Dieopipa. 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 downward ; the malar ascending in front to the lachrymal in a flattened perpen- 
dicular plate; facial surface of the maxillaries minutely perforated ; mastoid portion of auditory bull 
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. Jaculine. One premolar above. Grinding-teeth rooted. Cervical vertebra free, metatarsals 
separate. Hind feet with five developed digits. Tail sparsely haired. Nearctic. [Oue genus,—Jaculus.] 
“B. Dipodine. Premolars present or absent. Grinding-teeth rooted. Cervical vertebra more or 
less ankylosed. Metatarsals united in a cannon-bone. Hind feet with only three digits functionally 
developed. Tail thickly haired, often tufted. Palzarctic and Ethiopian. [Three genera,—Dipus, Alactaga, 
Platycercomys. , 
“C. Pedetine. One premolar above and below. Grinding-teeth rootless. Cervical vertebra free, 
Metatarsals separate. Hind feet with four well-developed digits, with short broad hoofslike nails. Tail 
bushy throughout, not tufted. Ethiopian.” [One genus,—Pcdetes.] 
