MILLER AND GIDLEY: SUPERGENERIC GROUPS OF RODENTS 431 



are still in flux. The Euciliata, advancing from this condition, 

 have become stereotyped in a definitely binucleate state with 

 secondary nuclear specialization. 



Classification of Ciliata 

 Protociliata 

 Opalinidae 



Protoopalina 

 Opalina 

 Euciliata 



ZOOLOGY. — Synopsis of the supergeneric groups of Rodents.^ 

 Gerrit S. Miller, Jr., and James W. Gidley, U. S. 

 National Museum. 



Work on the taxonomy of the Rodents, living and extinct, has 

 occupied much of our time during the past four years. This 

 paper contains a brief synopsis of the results. 



The classification which we have adopted is based on the fol- 

 lowing conception of the evolutionary course followed by the 

 order during its development. This course has been mainly 

 conditioned by the mechanical problem of strengthening a chew- 

 ing apparatus in which the unusually important cutting func- 

 tion of the incisors is strongly contrasted with the grinding func- 

 tion of the cheekteeth; the highest degree of efficiency to be 

 given always to the incisors and in most instances to the cheek- 

 teeth as well. The problem has been solved by five sequences of 

 correlated changes in the masseter muscle and the bones to 

 which this muscle is attached. All of these sequences could 

 originate from the structures present in a generalized mammal, 

 but there is no evidence that any rodent during its development 

 has passed from one to another. The groups characterized by 

 the various sequences are therefore natural. We have treated 

 them as superf amilies : the Sciuroidae, Myoidae, Dipodoidae, 

 Bathyergoidae, and Hystricoidae. Of the secondary problems the 

 most conspicuous has been the strengthening of the cheekteeth. 

 These teeth, however unHke their structure in extreme in- 

 stances may appear, have all been developed from some primi- 

 tive, low-crowned, tritubercular type not essentially different 



1 Published by permission of the Secretary of the Smithsonian Institution. 



