No. 377.-] THE ORIGIN OF THE MAMMALIA. 333 
We had rather anticipated from our knowledge of the earliest 
Stonesfield mammals that their reptilian ancestors would be 
very small. The large size of these Permian theriodonts, how- 
ever, is not incompatible with the hypothesis that smaller 
and less specialized 
members of the 
group may have con- 
stituted a persistent 
phylum. 
`The reéxamina- 
tion of the jaws of 
: 13.— Triconodon, a typical triconodont from the Upp 
the Upper Triassic se nies ceed Beds of England. (Original from speci- 
ens in the British Museum.) 
Dromotherium and 
Microconodon fails to reveal any evidence of a composite 
nature, that is, so far as it is possible to determine; the jaws 
consist of single bones, but they are so small that this evidence 
is not conclusive. The position of the Protodonta, there- 
fore, appears to be unaffected by Seeley s discoveries. - The 
Gomphodontia of Seeley are likewise separated from the 
Multituberculata of Cope by the composite nature of the jaw, 
but it remains to be seen how far the more recent multituber- 
culates, such as Polymasto- 
don, which certainly have the 
single jaw of the mammals, 
may have retained other rep- 
tilian characters in the skull. 
Fic. 14.— Jaw of Microconodon tenuirostre, a 
protodont from the Upper Triassic of North We reach the general con- 
Carolina. A, supposed rudiment of angle. ; ; £ 
clusion that the Theriodontia 
constitute a group which contains practically all the primitive 
characters of the Mammalia in the skeleton and teeth, and that 
no other reptiles or amphibians approach so near the hypo- 
thetical promammal. The explanation of the presence of 
amphibian characters in the soft parts of the existing Mammalia 
appears to be that the promammal sprang from primitive 
reptiles which preserved a number of still more primitive 
amphibian or stegocephalian characters. 
