No. 377.] THE ORIGIN OF THE MAMMALIA. 313 
which prove to be unrelated to the theromorphs and still less 
to the mammals. 
The most important series of explorations in the Karoo Beds 
of South Africa, directed by Professor Seeley, thus turn our 
thoughts upon the origin of the mammals into the old channel 
considered by Owen, in spite of his indefinite views of evolu- 
tion. The animals first described by him as Cynodontia and 
later as Theriodontia in 1876, both terms being given in full 
recognition of the resemblances which these animals presented 
to the Mammalia in their teeth, are, thanks to these explora- 
tions, very much more fully known. Seeley’s successive memoirs 
Fic. 1. — Jaw of Dromatherium sylvestre, a protodont from the Upper Triassic of North Carolina. 
detail many of their numerous points of likeness to the recent 
and extinct Mammalia. These memoirs may therefore be 
reviewed in connection with previous speculations as to the 
ancestry of the mammals. We may critically consider the 
question of resemblances, in order to determine how far we are 
justified in supporting the hypothesis that the mammals sprang 
from the theriodont reptiles. 
Seeley (1896, pp. 183, 184) has recently referred the species 
Labyrinthodon riitimeyeri of Wiedersheim to a new genus, 
Aristodesmus. After pointing out the numerous resemblances 
of this form to the monotremes, he closes as follows : 
In conclusion, the author argues that the points of structure are so few 
in which monotreme mammals make a closer approximation to the higher 
mammals than is seen in this fossil and other Anomodontia that the mono- 
treme resemblances to fossil reptiles become increased in importance. He 
believes that a group Theropsida might be made to include Monotremata 
and Anomodontia, the principal differences (other than those of the skull) 
being that monotremes preserve the marsupial bones, the atlas vertebra, and 
certain cranial sutures. Ornithorhynchus shows prefrontal and postfrontal 
bones, and has the malar arch formed as in anomodonts. 
