292 
It must be understood that our knowledge of the mammals of the 
Lance Creek and related formations is of a very unsatisfactory kind. 
With few exceptions, all that is known of these animals has been derived 
from their teeth, not found in place in the jaws, but scattered singly 
through the rocks. Better known are the Jurassic mammals, for of these 
many jaws have been secured. Recently considerable light has been 
thrown on the marsupials of the Lance Creek and Fort Union formations 
through the discovery of the skull and some parts of the skeleton of 
Ptilodus (Gidley, Proc. U. S. Nat. Mus., xxxvi, p. 611). The other genera 
await elucidation. Osborn’s statement of the situation may be accepted 
(Evolution of the mammalian molars, 1907, p. 95) : 
It is possible that, besides Marsupials, we find here Insectivores, 
primitive Carnivores, and the ancestors of ancient Ungulates; but it is ob- 
vious that the determination of relationships from such isolated materials 
is a very difficult and hazardous matter. 
Notwithstanding this appreciation of the situation, Professor Osborn 
has ventured (op. cit., pp. 12, 22, 115) to refer his Trituberculata, Marsh’s 
Pantotheria, to the infraclass Placentalia. No adverse criticism can be 
made on this procedure, in case its tentative character is understood. 
Now, while this uncertainty reigns regarding the systematic relation- 
ships of the mammals of the Lance Creek and related deposits, the case is 
different as soon as attention is given to the mammals of the Puerco, Tor- 
rejon, and Fort Union. Some of them betray by their tooth succession 
and other characters that they are true placentals. Many of them may be 
referred with confidence to orders and families that continued long after- 
wards, some of them probably to the present day. 
That a considerable gap existed between the mammals of the Lance 
Creek formation and those of the Puerco and Torrejon is evident from the 
state of development of the teeth. Osborn, speaking of the teeth of the 
Upper Cretaceous mammals [Lance Creek] says (Bull. Amer. Mus. Nat. 
Hist., v., 1893, p. 321) that in none of the molars hitherto described and 
in none of his collection of about 400 teeth and some jaws was there any 
trace of the hypocone, or posterior internal tubercle. Nor was any hypo- 
cone recognized in the genera described by him in 1898 (Bull. Amer. Mus., 
Nat. Hist., x, p. 171). Undoubtedly, however, the hypocone is sometimes 
present in a rather rudimentary condition, as I have observed in teeth 
shown me by Mr. Gidley, of the U. S. National Museum. Nevertheless, 
