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
ZOOLOGY. — Synopsis of the supergeneric groups of Rodents.^
Gerrit S. Miller, Jr., and James W. Gidley, U. S.
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.