38 WILLIAM K. GREGORY 
noids, occipital condyles, interparietal, posttemporal canal (cf. 
Echidna), etc., than Sesamodon, with its opossum-like zygoma 
and auditory groove, its infraorbital canal, its nostrils, its lower 
jaw, its dentition? 
The mammalian affinities of the Theriodonts are thrown into 
even clearer emphasis wheg we compare other extinct reptiles 
with the mammals. How wide is the structural gap between 
mammals, on the one hand, and Pelycosaurs, Cotylosaurs, lizards, 
Rhynchocephalians, Chelonians on the other. And if we extend 
our comparisons to the post-cranial skeleton of the Theriodonts" 
we again find that, after setting aside generic specializations 
Fig. 25 Skull of Sesamodon browni, the most mammal-like known Cynodont. 
(Broom P.Z.S. 1911, p. 914, text-fig. 179.) 
we have left a great majority of characters favoring the view 
that the mammals sprang from Cynodonts of some sort. The 
seapulo-coracoid of Gomphognathus, for example, furnishes the 
complete key” to the derivation of the mammalian from the 
reptilian type; the humerus also is in every respect transitional 
between the Permian reptile and the mammalian types. Even 
the true Anomodonts, far removed as they are from direct rela- 
tionship with the mammals, show an essentially mammalian manus 
and pes.! 
11 See the discussion in The orders of mammals, pp. 118-119. 
12 Tbid:, p. 119. 
13 Tbid., pp. 439-453. 
