No.2.] THE HARD PARTS OF THE MAMMALIA. 267 
from vertical in the primitive forms (Sciuridze) to oblique in the 
genera with prismatic molars, is simple enough. The inferior 
crowns when closely appressed to the superior, and drawn _pos- 
teriorly in the direction of the long axis of the jaw, press and 
strain the teeth in the two directions mentioned. The develop- 
ment of the long prismatic crowns which has proceeded under 
these circumstances, has been undoubtedly affected by the 
pressure and strain, and the direction we find has been the 
result. 
Figure 88. — Ischyromys typus Leidy, from the White River beds of Colorado; 
original; from the Report U. S. Geol. Surv. Terrs.: @, 6, ¢, cranium; d, mandible 
from above. 
The seventh effect is in the detailed structure of the teeth 
themselves. Beginning with short crowns with simple trans- 
verse crests (Psittacotherium and Sciuride, Figs. 84, 88), we 
reach through intermediate forms, crowns with vertical laminze 
of enamel, which sometimes divide the crown entirely across 
(Chinchillidze, Caviidae, Castoroididae) or appear only on the side 
of the crown, through the continued coalescence of the prisms 
of which each molar crown is composed (Arvicola). In many 
instances the crowns increase in transverse at the expense of 
