MORPHOLOGY OF THE VERTEBRATE SKULL 25 
unappreciated. Kingsley (’00), Gadow (’01), Gaupp (11) and 
others have held that the Theriodonts, being monimostylic, were 
definitely excluded from the mammalian ancestry, because, from 
embryological evidence, the mammals are inferred to have 
descended from forms with a freely movable quadrate. 
The Theriodonts were apparently first considered as favoring 
the ‘quadrate-incus’ doctrine in 1909-1910 by the present writer 
(10) in the following conclusions: 
1. In Therocephalians and Cynodonts the progressive enlarge- 
ment of the ascending ramus of the dentary and the progressive 
reduction of quadrate, articular and angular were regarded as 
adaptively correlated processes, tending on the one hand towards 
the formation of a new squamoso-dentary joint and on the other 
hand to a decrease in suspensorial functions of the old see 
articular joint. 
2. From the conditions in Cynognathus, Trirachodon, etc., it 
seemed plain that the new joint, when established, must hee 
been not far in front of the old joint (fig. 16); that there was more 
or less slip between 'the dentary and the angular (’10, p. 137); 
and that the new and the old joints long functioned together, 
all these relations being prophesied, as it were, although not 
attained in known Cynodonts. 
3. A certain groove in the base of the skull of Cynognathus 
(fig. 18) was shown (Joe. cit., p. 121) to have identical topographic 
relations with the auditory groove of mammals; it was, there- 
fore, probably homologous with that structure and hence it was 
fair to assume that the tympanic cavity and tympanic mem- 
brane were closely associated with this groove and consequently 
lay below the reduced articular and quadrate (loc. cit., p. 122, 
fig. Mego: ty; p. 141), 
4. From these inferred relations of the tympanic cavity and 
membrane in Cynognathus, and from the fact that in ontogeny 
the tubo-tympanal cavity grows up around the auditory ossicles 
which arise outside of it, it was suggested (710 a, p. 126, fig. 3 B; 
10 b, p. 600) that phylogenetically this upgrowing of the tubo- 
tympanal sac (fig. 19) around the vestigial quadrate and articular 
may have caused them to share in its vibrations and thus to 
