50 W. H. F. ADDISON AND J. L. APPLETON, JR. 
incisor forms on the outer aspect of the mandible a marked 
rounded projection, directed upwards and backwards beneath 
the coronoid process, and sometimes extending slightly posteriorly 
beneath the sigmoid notech.- Almost directly opposite this 
projection on the mesial aspect of the mandible is the inferior 
dental foramen. This projection marks the position of the grow- 
ing end of the formative organs of the incisor in the adult. In 
the new-born animal it is not present, nor at the end of the 
first month. By the age of 23 months it may be recognized, 
and thereafter it increases in prominence and constitutes a 
very evident feature of the bone. This region of the growing 
end of the tooth is protected by the zygomatic arch, and also by 
the overlying muscles. 
The course of the upper incisor may also be readily followed 
in the prepared skull. Laterally it is covered with a thin rounded 
layer of bone. Mesially it forms an elevated, distinct ridge 
projecting markedly into the nasal fossa. In the adult the 
position of its basal or growing end is not so prominent as that 
of the lower incisor. As these incisor teeth are an indispensable 
part of the rodents’ existence their importance demands pro- 
tection from traumatism which might injure their growing pulp. 
Here in the upper incisors, this protection is afforded by a flange 
of the maxilla running parallel to the lateral wall of the cranium, 
as shown in figure 1, as well as being encased in a separate thim- 
ble-shaped recess of bone (fig. 3), beneath, and separated by 
a narrow interval from, the outer layer of the maxilla. These 
details are in harmony with Cope’s idea (’88) of the influence 
of the incisors in moulding the general topography of the rodent 
skull. 
The diastema in the upper jaw is always longer than in the 
lower (fig. 1). By reference to table 1 it may be seen that in 
the mature animal the upper is nearly twice as long as the lower, 
but that in the younger stages the difference is not so great. 
The upper hair-covered lips are infolded into the diastema, 
dividing the oral cavity into an anterior and posterior com- 
partment. This arrangement probably prevents the débris 
and splinters of gnawing from entering the main oral cavity. 
