314 THE AMERICAN NATURALIST. [VoL. XXXII. 
Aristodesmus, which suggests this link, is at present placed in the 
Procolophonia, a group separated from its recent association with Pareia- 
saurus, and restored to its original independence because it has two occipital 
condyles, with the occipital plate vertical and without lateral vacuities, and 
has the shoulder girdle distinct from Pareiasauria in the separate precoracoid 
extending in advance of the scapula. 
A similar view is that of Mivart, who removes the monotremes so far 
from the marsupials and placentals as to conclude that they arose from 
sauropsidan ancestors, while the higher mammals, marsupials and placentals 
sprang independently from Amphibia-like stem forms.! 
I. CHARACTERS OF THE PROMAMMAL. 
It is obvious that to establish a point of connection we should 
first take characters furnished by the most ancient members of 
the class of mammals and picture the mammalian prototype or 
promammal, 
As regards the teeth, I aade such an attempt in 1893 
(Rise of the Mammalia) in the following terms: 
The Permian Sauro-mammalia (Baur) with a multiple succession of 
simple conical teeth divided into: (1) Theromorpha, which lost the succes- 
sion and in some lines acquired a heterodont dentition and triconid single- 
fanged molars; (2) Promammalia. The hypothetical Lower Triassic 
Promammalia retained a double succession of the teeth; they became 
heterodont, with incipient triconid double-fanged molars, the dental formula 
approximating 4, I, 14-5, 8. They gave rise has three groups: (a) The Proto- 
theria, whic pidly through the t lar into the multitubercular 
molars in the line of inltitehercaluses and more slowly into trituberculy, 
and its later stages in the line of monotremes. (4) They gave off the 
Metatheria, or marsupials, and finally (c) the Eutheria, or placentals. 
In the same address I took very positively the position that 
the simple reptilian cone is the ancestor of the multitubercular 
as well as of the tritubercular dental types, and that the multi- 
tuberculate teeth observed in the Triassic were not primitive, 
but had precociously passed through a tritubercular stage. I 
derived the characters of the promammal from a study of all 
the known Jurassic Mammalia. The inference as to the multiple 
succession of the teeth I subsequently based upon the recent 
embryological demonstration that all living mammals are diphyo- 
dont and sprang from polyphyodont ancestors (a principle that 
1 Proc. Roy. Soc., vol. xliii, p. 372. 
