440) University of California Publications in Zoology — (Vou. 17 
though never, apparently, forming a true sagittal crest (the degree 
of their approach is different in different species) ; the mastoid pro- 
cesses tend to grow laterad more rapidly than do the audital tubes; 
the lambdoidal ridge undergoes marked development; the skull 
becomes more flat, less round, changing from the more squirrel-like 
form of early youth to the more specialized Aplodontia type of full 
maturity ; all processes become accentuated, and the angular process 
of the mandible undergoes a considerable transverse expansion, its 
development proceeding at such a rate that the width of the mandible, 
measured along the axis of this process, increases faster proportion- 
ally than does the length of the mandible. Through all these changes 
the distance anteriorly across the palate between the alveoli of the 
fourth premolars remains practically constant. 
The permanent teeth are long-crowned and as soon as they become 
somewhat worn afford no elue to age. The tooth formula of Aplo- 
=o las ONOM ARO ens 
dontia is 1 oS an Bae 3 
specimens is as follows: Superior, milk P*, milk P*, M’, M*, M*, 
Ore Tooth eruption in available 
permanent P*, permanent P*; inferior, milk P,, M,, M., M., per- 
manent P,. 
The fourth premolars (P*, P,), are somewhat less specialized in 
the deciduous dentition than in the permanent. They are brachyo- 
dont, somewhat tubereulated, and have deep enamel lakes, in the 
former; hypsodont, with tubereulation obscure, and with shallower 
enamel lakes, all trace of which is soon lost by wear, in the latter. 
The considerable variation in size of cranium, as well as in the 
weight of its bars and processes, noticeable in series of fully adult 
skulls from the same general locality, indicates that slow growth may 
continue throughout life. 
2. SexusAL DIFFERENCES 
Aside from a not well-marked tendency toward larger measure- 
ments on the part of the males (which, in specimens of Aplodontia 
rufa phaca measured, average eight per cent longer than the females), 
and the presence of a series of conspicuous ventral markings about 
the mammae of summer females, practically no differences due to 
sex can be made out externally. There are in this genus three pairs 
of mammae. About each, in females in summer pelage, is a nearly 
circular avea of black or dark brown hair ten to twenty millimeters 
