Origin of the Dentition of the Rodentia. 5 
. 
The peculiarities of the rodent dentition consist, as is well known, 
in the great development of the incisors; the loss of all but one, 
or rarely of two, of the premolars, which leaves a wide diastema ; 
and the posterior position of the molar teeth, as relates to the rest 
of the skull. A peculiarity which belongs to the highest types of 
the order is the prismatic form of the molars, and the deep inflection 
of their always transverse enamel folds, both laterally and verti- 
cally. A peculiarity of the masticating apparatus, which is the 
basis of distinction from the Bunotherian order, is the lack of 
postglenoid process, and the consequent freedom of the lower jaw 
Fig. 2. Calamodon ngs Pa Cope, lower jaw, left 
ramus, one-third natural Original, from asatch 
Eocene, of Wyoming. Fig. a,e ecg view ; b, super- 
ior, e-d, inferior molar; ¢, exterior; d, poatenion views. 
to slide backward and forward in mastication. Appropriately to 
this motion the condyle of the mandible is either subglobular, or 
is extended anteroposteriorly, and the glenoid cavity is a longi- 
tudinal instead of a transverse groove. 
he mechanical action of the development of the rodent den- 
tition has been as follows: The first factor in the order of time 
and importance was the increasing length of the incisor teeth. 
are disappearing from the Tzeniodonta, while the supposed canines of 
the lower jaw of Megalonyx and allies may be trueincisors. This is 
rendered probable by the genus Diadomus of Ameghino, where the large 
Canine-like teeth are found close together at the soles bd mandibuli, 
like the incisors of Teeniodonta and Rodentia. 
