GEOLOGICAL SUCCESSION OF .MnLAi: TYI'KS 95 



Mr. Gidley's observations upon American Jurassic mammals are given 

 on pages 219-223. 



4. UPPER CRETACEOUS MAMMALS. 



(See also pp. 105, 115, 11<>.) 



A very long geological interval, representing an enormous period 

 of time, separates these mammals from those of the Upper Jurassic. With 

 few exceptions our entire knowledge of the upper Cretaceous mammals is 

 derived from collections made in the Laramie formation of the Kocky 

 Mountain region. These collections again embrace the two great dental 

 types of multituberculates (Fig. 55) and trituberculates. All the multi- 

 tuberculate types of teeth and jaws are too far specialized to give origin 

 to any of the modern divisions of mammalia, either placental or marsupial. 



The tf if /'!><' /'dilates (Fig. 47; see also pp. 115, 116) are represented 

 in the American Museum collection by a series of isolated upper and 

 lower molars, also by a number of fragmentary jaws ; and among the latter 

 are some in which the angle is apparently inflected, indicating Marsupial 

 relationship. It is possible that, besides Marsupials, we find here Insec- 

 tivores, primitive Carnivores, and the ancestors of ancient types of 

 Ungulates ; but it is obvious that the determination of relationships 

 from such isolated material is a very difficult and hazardous matter. 



Nevertheless, the teeth of trituberculate type are of extraordinary 

 interest, since they lend strong support to the theory of the unity of origin 

 of the molar types of the higher mammals from a tritubercular stem, instead 

 of from a multitubercular, as has been suggested. 



The upper molars so far as known, are of the simple, generally low- 

 crowned tritubercular type as distinguished from the high sharp cusped 

 crowns of the Jurassic molars. The main triangle of cones, or trigon, 

 is of symmetrical form. In the majority of specimens, the largest 

 cone is the main internal one, or protocone (Fig. 47, A, D), while the 

 external cones, paracone and metacone, are symmetrical, invariably of 

 smaller diameter, and as a rule less elevated. These proportions sustain 

 the 'palseontological' theory that the reptilian cone is internal* rather than 

 the embryological and premolar analogy theories, that the reptilian cone 

 is antero-external in the true molars as in the prernolars. The outer wall 

 of the crown is reinforced by a cingulum, on which secondary spurs or 

 cuspules are developed, sometimes of very large size, more or less homo- 

 logous with the 'styles' of the tertiary types of molars (e.g. Fig. 68). 



Another feature recently discovered among Jurassic mammal teeth is 

 the presence of intermediate tubercles or conules on the trigon. A very 



* [This may be a non sequifnr, because the great elevation and size of the paraconid in 

 HI, Al, Fig. 47, need not be interpreted as indicating its relative antiquity. ED.] 



