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 foi'ination 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, 



