268 COPE. [Vot. III. 
their longitudinal diameter (Castor, Lepus). The vertically 
laminated structure is evidently due to the crowding together of 
ae 
transverse crests by the same pressure 
which has given the crowns their oblique 
direction. In many genera the length- 
ening of the crown has included the 
lengthening of the longitudinal connec- 
tion between the transverse crests, as 
in Arvicola, Castor, and Hystricidze 
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 Am- 
“4 blyrhiza and Saccomyide. In others, 
/ connection between the prisms has been 
/ lost by coenogeny, as in Chinchillidze 
| and Caviide generally. The latter fam- 
(ilies display also the greatest amount 
Figure 89.— Teeth of Hy- of crowding (Fig. 89). 
Mihi oe pas ee. A peculiarity of the plication of ro- 
after Leidy: @, fragment of Gent molars I am unable to explain as 
superior incisor; 4,the shaded yet on mechanical principles. In ge- 
portions represent parts of in- nera which are isognathous, the inflec- 
ferior molars found. . : : 
tions are of equal depth on opposite sides 
of both superior and inferior molars. In anisognathous genera 
the inflections are more numerous and profound on opposite 
sides of the molars of the respective jaws. Anisognathism in 
rodents is generally as shown by Ryder, of the type where the 
Figure go.— Molars of Rodentia compared with the haplodont (@) and seleno- 
dont (@) dentitions: a, Arvicola, first inferior molars; 6, do. profile; e, Thomomys 
crowns; /, do, profile; from Ryder. 
