432 MILLER AND gidley: supergeneric groups of rodents 



from that present in the Eocene Paraniyidae and in Hving species 

 of Sciurus. During the adjustment of the cheekteeth to increas- 

 ingly heavy fore-and-aft grinding motion, a process which has 

 taken place in most members of the order, the crown height has 

 been augmented, while the original tubercles and lophs have been 

 made more efficient by (a) increase in complexity, and (b) con- 

 version into transverse ridges and specialized enamel plates, usu- 

 ally with reduction in the number of elements present. In each 

 superfamily the characteristic modifications in the muscles and 

 skull were begun in connection with the development of the in- 

 cisors. Mechanical improvement of the cheekteeth came later. 

 All rodent teeth have been developed from an essentially uniform 

 original type under the influence of practically identical mechani- 

 cal forces. Parallelism in highly specialized dental structures 

 between genera and species which are not closely related is 

 therefore frequent enough to be one of the noticeable peculiari- 

 ties of the order. The history of development extends so far into 

 the past that the essential features of structure are modernized 

 in the oldest known Eocene rodents. No extinct member of 

 the order has yet been found which can be regarded as ancestral 

 to any considerable number of subsequent forms. 



The order Rodentia may be defined, as follows: Terrestrial and 

 fossorial (occasionally arboreal or semiaquatic) placental mammals 

 with both brain and placentation generalized in type ; feet vmguiculate ; 

 elbow joint always permitting free rotary motion of forearm; fibula 

 never articulating with calcaneum; niasseter muscle highly specialized, 

 divided into three or more distinct portions having slightly different 

 functions; cecum without spiral fold; dental formula not known to 

 exceed i }, c f pm f, m f = 22 permanent teeth; incisors scalpriform, 

 growing from persistent pulp, the enamel of the upper tooth not ex- 

 tending to posterior surface; distance between mandibular and maxil- 

 lary toothrows approximately equal, both pairs of rows capable of par- 

 tial or complete opposition at the same time, the primary motion of the 

 lower jaw in mastication longitudinal or oblique. 



Superfamily SCIUROIDAE 



Masseter lateralis superficialis with anterior head distinct, this por- 

 tion of the muscle not attached to any part of the zygoma except occa- 

 sionally to a point at extreme base of zygomatic plate; zygomatic plate 



