462 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
rank, as was done by Professor Gill in 1872, probably for the first time, 
although Carus had already proposed Pedetina, Dipodina, and Jaculina, thus 
making the three groups of co-ordinate value. 
Differing, as I do, with the majority of writers, who associate Zapus more 
intimately with Dipus and Pedetes than with any typical Muride, it may be 
well to compare or contrast the characters that bear upon the case. Certain 
modifications of the skull and of the metatarsus, and the dental formule, are 
chiefly concerned. 
The four families Murida, Zapodide, Dipodide, and Pedetide agree in 
the completeness of the clavicles, anchylosis of the tibia and fibula, particular 
condition of the angle of the mandible, absence of postorbital processes, and 
other features characteristic of, or normal in, the Murine series of Rodents. 
It is highly characteristic of the Murid@, as now usually accepted, to possess 
.3=3 
°=* molars, without premolars; the only exceptions, as far as known, being 
the genus Sminthus, which has {=} premolars, and the genus Hydromys, 
2 
which has only 3-3 molars (Alston). Zapus departs from the rule in having 
i‘? premolars, aud in so much approaches Dipodidae and Pedetide. But these 
last two families differ between themselves in respect to the premolars, these 
being absent, or present above only, in Dipodide, and present above and below 
in Pedetide. Uence the condition of the premolars fails to be decisive. The 
state of the molars is likewise not diagnostic. Excepting the genus Hydro- 
mys, the molars are 2=3 in all of the families in question; and they are indif- 
ferently rooted or rootless in Muride, rooted in Zapodide and Dipedida, 
rootless in Pedetida. 
It is highly characteristic—almost diagnostic—of Murideé to possess a 
particular construction of the anteorbital foramen; this-aperture being nor- 
mally a pyriform slit of moderate or small calibre, bounded externally by a 
broad plate of the maxillary. Zapus, Dipus, and Pedetes all depart unequiv- 
ocally from this in having the same opening large or very large, rounded, 
and (always?) supplemented with a nick or additional foramen below the 
main aperture. Associated with this condition of the foramen, we find a 
special state of the zygoma, which is more than ordinarily depressed, and the 
malar element of which is prolonged up the maxillary to effect suture with 
the lachrymal; whereas, in typical Muride, the malar is a mere splint, joining 
extended maxillar and squamosal processes. There are some other features, 
such as shortness and breadth of the brain-case and condition of the auditory 
