10 Origin of the Dentition of the Rodentia. 
closely appressed to the superior, and drawn posteriorly in the 
direction of the long axis of the jaw, press and strain the teeth 
in the two 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 them- 
selves. Beginning with short crowns with simple transverse crests, 
(Psittacotherium and Sciuridæ, Figs. 1 and 5), we reach, through 
intermediate forms, crowns with vertical lamine of enamel, which 
sometimes divide the 
crown entirely across 
(Chinchillidæ, Caviidæ, 
Castoroididæ), or appear 
> only on the ends of the 
crown, through the con- 
prisms of which each 
molar crown is composed 
(Arvicola). In many 
instances the crowns in- 
Fie Chirox plicatus Cope, palat $ 
iry trom ea of Ne ras aia R emna aS oeer crease ın transverse at 
best gil “ee oe Mexico. From American Natu- the expense of their lon- 
gitudinal diameter (Cas- 
tor, 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 direction. In many 
genera the lengthening of the crown has included the lengthen- 
ing of the longitudinal connection between the transverse crests, 
as in Arvicola, Castor and Hystricide generally. In others this 
connection has not been continued, so that the crown is composed 
of prisms which are separate to near the base, as in Amblyrhiza 
and Saccomyide. In others, connection between the prisms has 
been lost by ccenogeny, as in Chinchillidee and Caviide generally. 
The latter families display also the greatest amount of crowding 
(Fig. 6). 
A peculiarity of the plication of rodent molars I am unable to 
explain as yet on mechanical principles. In genera which are 
isognathous, the inflections are of equal depth on opposite sides 
of both superior and inferior molars. In anisognathous genera 
