DENTAL SYSTEM 31 
which the teeth were in the form of almost a quite simple cone ; 
such a presumably primitive type of dentition being apparently 
retained among some existing Edentates, like the Armadillos, while 
it is possible that we should regard the dentition of the existing 
Cetacea (Fig. 2) as a reversion to the same primitive type. None of 
the Mesozoic mammals at present known exhibit this simple 
conical type of teeth, although we have an approximation to it in 
the extremely generalised genus Dromatherium. Starting then 
Fia. 4.—Molar teeth of Mesozoic Mammals(enlarged). Triconodont type—1, Dromatherium ; 
2, Microconodon ; 8, Amphilestes ; 4, Phascolotheriwm ; 5, Triconodon. Tritubercular type—6, 7, 
Spalacotherium ; 10, Asthenodon. Tubercular sectorial type—8, Amphitheriwm ; 9, Peramus ; 11- 
13, Amblotherium; 14 (2) Amblotherium. pr, Protocone ; hy, hypocone; pa, paracone; me, 
metacone, in the upper teeth; and protoconid, hypoconid, paraconid, and metaconid in the 
lower, 6 and 15 are upper molars, and the rest lower molars. (After Osborn.) 
from this presumed simple cone it appears that the teeth of Dromu- 
therium (Fig. 4,1) present the first stage towards trituberculism, the 
crown of each tooth having one main cone, with minute lateral 
cusps, and the root being grooved. In the next or true Tricon- 
odont stage (Fig. 4, 3.5) the crown has become elongated antero- 
posteriorly, and consists of one central and two lateral cones or 
cusps, while the root is divided. From this the transition is easy to 
the tritubercular type, in which the three cusps, instead of being 
placed in a line, are arranged in a triangle; the upper teeth (Fig. 
