No. 2.] THE HARD PARTS OF THE MAMMALIA. 263 
sor is rudimental in Calamodon, and Marsh has shown the same 
thing in Tillotherium. In both genera the second incisor is the 
effective tooth. The third is apparently present in Calamodon 
(Fig. 84), but the homology of this third tooth is not yet fixed. In 
Tillotherium the third incisor is apparently wanting. In Psittaco- 
therium the first incisor tooth is present and effective, but the 
second is larger. It is not certain whether these are first and 
second, or second and third incisors. If we allow Esthonyx 
to decide the question, the large second tooth is truly the 
second incisor, for in that genus the first incisor is small, and 
the third is rudimental. With present information, then, the 
inferior incisor of the Rodentia is the second of the Mammalian 
series, ! 
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 leave 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 vertically. 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 to slide backwards and forwards in 
mastication. Appropriately to this motion the condyle of the 
mandible is either subglobular, or is extended anteroposteriorly, 
and the glenoid cavity is a longitudinal instead of a transverse 
groove. 
The mechanical action of the development of the rodent 
dentition has been as follows. The first factor in the order of 
time and importance was the increasing length of the incisor 
teeth. Those of the lower jaw closed behind those of the upper 
in the progenitors of the Rodentia (e.g. Esthonyx) as in other 
Mammalia. Increase of length of these teeth in both jaws 
1] have regarded (Naturalist, 1884, April and earlier) the Tzeniodonta as the 
ancestors of the Edentata. The objection to this view is the supposed absence of in- 
ferior incisors in the latter. But the middle incisors have disappeared from some of the 
Tzniodonta, while the supposed canines of the lower jaw of Megalonyx and allies may 
be true incisors. This is rendered probable by the genus Diadomus of Ameghino, 
where the large canine-like teeth are close together at the symphysis mandibuli, like 
the incisors of the Tzeniodonta and Rodentia, 
