On the Affinities of Tritylodon. 75 



there is a widening of the snout by the roots of the teeth which 

 have been regarded as the incisors. The facts that the front teeth 

 are separated by an interspace ; that the incisor roots extend into 

 the maxillary bones ; and that in Theriodonts evidences are some- 

 times apparently found of a successional canine behind the large 

 canine led Seeleyto doubt whether the large front teeth in Tritylodon 

 " ma} 7 " not be regarded as canines comparable to the canines of 

 Theriodonts, rather than as incisors comparable to the incisor teeth 

 of Mammals like Eodents." The occurrence of an interspace 

 between the front incisors is met with in a number of Mammals, 

 and the fact of the incisor roots extending into the maxillary bone 

 is of such common occurrence in Mammals that it may be said to be 

 the almost invariable rule if the incisors are large. As in no Reptile, 

 Theriodont or other, are large incisor teeth known which pass back 

 into the maxillary bone, the occurrence of them in Tritylodon is 

 rather to be regarded as an evidence of the mammalian affinity of 

 the genus. The swelling of the snout, caused by the teeth roots, 

 does not seem to be a character of much importance ; but there is 

 no difficulty in pointing out a number of Mammals in which it 

 occurs — both carnivorous and herbivorous. 



Another of Seeley's arguments in favour of the Theriodont affinity, 

 is that "the nares are terminal in Tritylodon and in Theriodonts" ; 

 but as the nares are also terminal in Mammals this fact proves 

 nothing. It will be pointed out, however, presently, that the nares 

 in Tritylodon are very different from those of the Theriodonts. 



Seeley finds another Theriodont character in the posterior nares. % 

 He says : " The posterior nares are conditioned as in Theriodonts, 

 opening between the hinder molar teeth. This character is not 

 mammalian. ... As far as the evidence goes the posterior nares 

 are Theriodont." It is difficult to understand how the conclusion 

 was arrived at that the opening of the posterior nares between the 

 hinder molar teeth is not a mammalian character. Among Eodents 

 — the very Mammals with which Seeley formerly placed Tritylodon — 

 it is such a common character that it might be regarded as the rule. 

 But the character is by no means confined to Eodents. It is met 

 with in forms as different in other respects as Petrogale, Procavia, 

 Palceomastodon, Eqtttis, Ovis, Coryphodon, Uintathermm, and Galeo- 

 pithecus. In fact, the list of Mammals in which the internal nares 

 open between the posterior molars can be extended to almost any 

 required length, and Mammals fulfilling the condition can be found 

 in most of the orders. It will thus be seen that the position of the 

 posterior nares in Tritylodon is similar to that seen in a large 

 number of Mammals. On the other hand, no Theriodont is known 



