92 TEETH OF THEROMORPHA CHAP. 
maxillae and the premaxillae above, is a sine qua non tor 
mammalian comparison. In the more basal Theromorpha the 
teeth are not so limited im position. Finally, to complete the 
remarkable mammalian resemblance of the teeth of these reptiles, 
it must be mentioned that in Zvritylodon and Diademodon the 
roots of the molars, as we may fairly term them, though not 
actually divided after the mammalian fashion, were deeply 
marked by a groove, which suggests an incipient division or a 
fusion of two distinct roots. Some of these facts of structure 
may now be considered in further detail. As to the incisors 
and canines, it is sufficient to say that the numbers of the former, 
and the shape of the latter, are in perfect consonance with a 
derivation of the Mammalia from this group. The molar series 
can be divided into premolars and molars, at least in so far as 
regards their shape; for the anterior teeth are often smaller and 
less complicated than those which follow, as is often the case with 
the two series in mammals. The molar series also consist of teeth 
in close apposition to each other and separated from the canines 
by a diastema, which is a character of mammalian teeth. The 
fact that im the reptile Cynognathus and the mammal J/yr- 
mecobius there are nine of these molar teeth in each half of each 
jaw is perhaps not a point upon which it is desirable to dwell 
with too much weight; but the general fact that the molars are 
further reduced in some genera of Theriodontia. than in that 
which has been mentioned, is clearly a matter of significance 
when the ancestry of the mammals is under consideration. 
The most interesting fact about the molar series in the 
Theriodontia is that we meet with the two types of molars that 
occur in the mammals. Cynognathus and other genera have 
molars which consist of a main cusp, and of one cusp before and 
one after the main cusp; in fact these teeth are triconodont as 
in certain early mammals, a state of affairs which is believed 
by the “trituberculists” (see p. 56) to have preceded the 
tritubercular tooth. There are also “ multitubercular” teeth, 
especially well developed in TZ'vritylodon, where they exactly 
resemble those of certain Multituberculata, and whose structure 
originally led to the placing of 7ritylodon among the mamunals of 
that group. If there is any question about the mammalian nature 
of this fossil, there remain several other Theriodontia in which 
the multituberculism is well marked. It is so in 7rirhachodon 
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