92 TEETH OF THEROMORPHA CHAP. 



maxillae and the premaxillae above, is a sine qua non for 

 mammalian comparison. In the more basal Theromorpha the 

 teeth are not so limited in position. Finally, to complete the 

 remarkable mammalian resemblance of the teeth of these reptiles, 

 it must be mentioned that in Tritylodon 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 in the reptile Cynognathus and the mammal Myr- 

 mecobiiis 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 Tritylodon, where they exactly 

 resemble those of certain Multituberculata, and whose structure 

 originally led to the placing of Tritylodon among the mammals 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 Trirhachodon 



