KINE TOGLNL SIS. 



351 



directions mentioned. The development 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. 



The seventh effect is in the detailed structure of 

 the teeth themselves. Beginning with short crowns 

 with simple transverse crests (Psittacotherium and Sci- 

 uridae, Figs. 106, 109), 



we reach through inter- MSL^^k t 



immmm b 

 mediate forms, crowns 



with vertical laminae of 

 enamel, which some- 

 times divide the crown 

 entirely across (Chinchil- 

 lidae, Caviidae, Castoroi'd- 

 idae) or appear only on 

 the side of the crown, 

 through the continued 

 coalescence of the prisms 

 of which each molar 

 crown is composed (Arvi- 

 cola). In many instances 

 the crowns increase in 

 transverse at the expense 



of their longitudinal diameter (Castor, Lepus). The 

 vertically laminated structure is evidently due to the 

 crowding together of transverse crests by the same 

 pressure which has given the crowns their oblique di- 

 rection. In many genera the lengthening of the crown 

 has included the lengthening of the longitudinal con- 

 nection between the transverse crests, as in Arvicola, 

 Castor, and Hystricidae generally. In others this con- 

 nection has not been continued, so that the crown is 



Fig. 109. Ischyromystypus'Lei&y, from 

 the White River beds of Colorado; orig- 

 inal ; from the Report U. S. Geol, Surv 

 Terrs.: b, cranium from below; J, man- 

 dible from above. 



