428 U. Ss. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
ones, with five toes. Tail not prominent, or wanting. External ear wanting. Incisors broad 
and flat with the incisive edge rectilineal. 
Of this remarkable group there are no species found as yet in America. It is distinguished 
from the Murinae and Arvicolinae by the distinct thumb, instead of this being a mere rudiment ; 
the hind legs as short as the front ones, instead of being from one and a half to twice as long; in 
having the tail either wanting, or not projecting beyond the fur, instead of being decidedly 
longer ; in having no external ear, and in having the anterior face of the incisors plane, with 
a straight cutting edge, instead of having it more or less rounded with a convex or curved 
cutting edge.! 
Sus-Famity DIPODINAE. 
The characters of this sub-family, as given by Wagner, are found inthe greatly elongated hind legs, fitted for taking long 
leaps ; the abbreviated fore legs, the long hairy tail and the large infra-orbital foramen. Additional features may, however, 
readily be derived from the conditions of the skull, teeth, and other points of structure. 
The incisors are considerably compressed laterally. The molars of opposite sides in each jaw 
are widely separated. Palatine portions of the intermaxillary, maxillary and palatine bones 
pretty much on the same plane. Incisive foramina and orbit large. Ante-orbital foramen very 
large, oval, as wide (or wider) below as above. The arch forming its outer border is not com- 
posed solely by the union of the upper and lower processes of the superior maxillary, as in the 
other divisions ; but the malar is extended along the postero-external edge of the maxillary arch 
to the anterior corner of the orbit, where it is joined to the lachrymal. The zygomatic arches 
are long and slender, dipping below the level of the palate. 
Not possessing any specimens of this family excepting species of Jaculus, I have been unable 
to define the general characteristics of the group more fully, especially as the two principal 
authors who have treated of the subject differ considerably in their views of its extent and 
features, and neither gave to it the limits which it now possesses. Thus, Wagner has included 
the genus Dipodomys, which belongs to an entirely different family. The genera of Dipodinae 
are, in fact, quite few ; Dipus, or the Jerboa being the principal old world representative. The 
only North American genus is Jaculus, with a very limited number of species—perhaps but one. 
'For a very elaboratedisc ussion of this group, see Brandt, Beitrage zur Kenntniss der augethiere Russlands, 1855, 
pages 196—217, 
