462 



MONOOUAPnS OF NORTH AMERICAN RODBNTIA. 



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

 althougii 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 Zapui more 

 intimately with Dipus and Pedetes than with any typical Muridee, it mny 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 formulas, are 

 chiefly concerned. 



The four families Murida, ZapodUla, Dipodida, and Pedetidte 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 o{i\\e Muridee,as now usually accepted, to possess 

 3^3 molars, without premolars; tiie only exceptions, as far as known, being 

 the genus Sminfhus, which has {5} premolars, svnd the genus Hydromys, 

 which lias only ^I, molars {Alston). Zapus departs from the rule in having 

 J55 premolars, and in so much approaches Dipodidce and Pedetida. But these 

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

 i>eing absent, or present above only, in Dipodida, and present above and below 

 in Pedetida. Hence the condition of the premolars fails to be decisive. The 

 state of the molars is likewise not diagnostic. Excepting the genus Hijdro- 

 mys, the molars are \^\ in all of the families in question ; and they are indif- 

 ferently rooted or rootless in Muridre, rooted in Zapodid<e and Dipodida, 

 rootless in Pedetulce. 



It is highly characteristic — almost diagnostic^-of Murida to possess a 

 jjarticular construction of the anteorbital foramen; this aperture being nor- 

 mally a pyriform slit of moderate or small calibre, bounded externally l)y a 

 broad plate of the maxillary. Zupus, 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 additi«)nal 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 laciirymal ; wliercas, in typical Murida, the malar is a mere splint, joining 

 extended maxillar and s(piamosal processes. There are some other features, 

 such as sliortncss and breadth of the brain-case and condition of the auditory 



