EXPLANATION OF PLATE 27 
Figs. 7, 8.—Anterior view of crania of sciurid type and aplodontid type, to illus- 
trate difference in infraorbital arrangement; X 0.77. Fig. 7, Marmota flaviventris 
sierrae, no. 15165, Mus. Vert. Zool.; fig. 8, Aplodontia rufa californica, no. 18663, 
Mus. Vert. Zool. 
Note in Marmota the broadly expanded fossa for the masseter muscle just 
laterad of the infraorbital foramen on either side; note also the compression and 
reduction in size of the infraorbital foramen in this genus; note, on the other 
hand, the generalized character of the region in Aplodontia, there being no 
provision for the masseter muscle anterior of the ventral border of the anterior 
root of the zygomatic arch, and the infraorbital foramen being comparatively 
large, much as in the Eocene rodent family Ischyromyidae. 
Figs. 9,10. Dorsal view of mandibles of aplodontid and sciurid, to illustrate 
different development of angular processes; X 0.66. Fig. 9, Marmota flaviventris 
sierrae, no. 15165, Mus. Vert. Zool.; fig. 10, Aplodontia rufa californica, no. 18663, 
Mus. Vert. Zool. : 
Note the lateral expansion of the angle in Aplodontia as compared with 
Marmota. 
[500 ] 
