462 MONOGRAPHS OF NORTIJ AMERICAN RODENTIA. 



rank, as was done by Professor Gill in 1872, probably for the first time, 

 although Cams had already proposed Pedetina, Dipod/na, 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 Muridce, 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 formula?, are 

 chiefly concerned. 



The four families Muridce, Zapodidce, Dipodida, and Pedetidce 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 3Iur idee, as now usually accepted, to possess 

 f^f molars, without premolars; tlie only exceptions, as far as known, being 

 the genus Sminthus, which has ~ premolars, and the genus Hydromys, 

 which has only l_ 2 2 molars {Alston). Zapus departs from the rule in having 

 i^i premolars, and in so much approaches Dipodida and Pedetidce. But these 

 last two families differ between themselves in respect to the premolars, these 

 being absent, or present above only, in Dipodida.', and present above and below 

 in Pedetidce. Hence 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 \~ in all of the families in question ; and they are indif- 

 ferently rooted or rootless in Muridce, rooted in Zapodidce and Dipodidce, 

 rootless in Pedetidce. 



It is highly characteristic — almost diagnostic — of Muridce 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 Muridce, 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 



