238 0. C. Marsh—New Orders of Jurassic Mammals. 
of the true molars differ from those in typical specimens of 
Dryolestes in having the cusps blunted, making the crowns un- 
usually short. Another important difference is that the tooth 
which may be regarded as the last premolar is so much larger 
than the rest of the series that it projects far beyond them. 
The line of the true molars is much curved outward. 
Some of the dimensions of this upper jaw are as follows: 
Space occupied by eleven posterior teeth,..---.- 12°0"" 
Extent of seven posterior teeth,......------.-- 8°0 
Projection of last upper premolar from jaw, ---- 270 
Ctenacodon serratus, Marsh. 
Additional remains of this interesting form have made clear 
some points in the structure of the lower jaw which the type 
specimen did not show.* The left lower jaw, represented in 
gure 2, agrees so closely with that specimen in size and gen- 
eral features, that it must be referred to the same species. ‘The 
ortion is very short, and nearly round. The inlet 
of the dental 
preserved are manifestly low generalized forms, without any 
distinctive Marsupial characters. Not a few of them show 
* This Journal, vol. xviii, p. 396, Nov., 1879. 
