824 STUDIES FOR STUDENTS 
Polyprotodonta: carnivorous or insectivorous forms of small 
size with a large number, 4-5, of incisor teeth and many 
molar and premolar teeth, 8-12 in opposition to 6—7 pos- 
sessed by recent forms. The premolars are simpler than the 
molars. 
The suborder is divided by Osborne into three groups, Pro- 
todonta, Triconodonta, and Trituberculata. The first, the Protodonta, 
is distinguished by having the premolars simple and conical in 
shape and the molars with a middle part slightly elevated above 
a posterior and an anterior accessory cone. The molar teeth are 
single-rooted, but there is a deep groove on each side that indi- 
cates the coming division of the root into two parts. The group 
is represented by two specimens only. These are from the Tri- 
assic rocks of North Carolina and consist of the lower jaws only. 
Dromotherium and Microconodon. 
Triconodonta: forms very similar to the last, but with a smaller 
number of molar teeth. The middle cone of the molars is better 
developed and the accessory cones are separated farther from 
the main one and have a much greater part in the function of 
mastication. The roots of the teeth are entirely separated. 
The forms are almost entirely from the Jurassic layers of Eng- 
land, and Wyoming. Typical genera are: 
Ampluilestes, Oolite from near Oxford, England. 
Phascalotherium, Oolite from near Oxford,, England. 
Tinodon, Upper Jurassic of Wyoming. 
Priacodon, Upper Jurassic of Wyoming. 
Dicrocynodon, Upper Jurassic of Wyoming. 
Trituberculata: small forms with many molar teeth, the crowns 
of which are supplied with three tubercles arranged in the form 
of a triangle with apex of the triangle pointing inwards in the 
upper teeth and outwards in the lower; the importance of these 
forms is best realized when we remember that this tritubercular 
arrangement of the tubercles of the teeth is the primitive type 
from which all the remaining types of mammalian dentition have 
been derived. The Amphitheride and the Amblotheride are the 
two most primitive families of the suborder. 
