STRUCTURE AND GROWTH OF INCISOR TEETH 49 
increase in the length of these teeth has been due to their con- 
tinued use, as believed by Ryder.” The effects of this increased 
elongation upon surrounding parts he described under several 
different headings, but reference will be made here only to one, 
viz., upon the shape of the glenoid cavity. “A peculiarity of 
the masticating apparatus is the lack of a postglenoid process, 
and the consequent freedom of the lower jaw to slide backward 
and forward in mastication. Appropriately to this motion, 
the condyle of the mandible is extended antero-posteriorly 
and the glenoid cavity is a longitudinal instead of a transverse 
groove.” 
Fig. 3 Thimble-shaped portion of the maxilla bone, in which the basal end 
of the upper incisor is located. X 2. 
The lower incisors are longer and more slender than the upper 
and extend far back in the mandible, beneath the lower molars, 
to near the sigmoid notch. The upper incisors are contained 
within the premaxilla and maxilla, the basal end occupying a 
thin-walled, thimble-shaped recess of bone (fig. 3) to be seen best 
in the disarticulated skull, and which is attached at only one 
limited region to the rest of the maxilla. In both upper and 
lower teeth, the intra-alveolar portions are longer than the 
extra-alveolar. When one compares the extra-alveolar lengths 
of the upper and lower teeth of the mature animal, the latter 
are always greater, and, as may be seen by reference to table 
1, the difference in lengths becomes greater with increased age 
and size. 
In both upper and lower incisors the bone is so contoured 
around their imbedded portions that their course may be easily 
recognized. The basal end or foraminal apex of the lower 
