MESOZOIC MAMMALS 



109 



has been gained, and what may still remain in store where little 

 thought of at present. A bed, apparently equally rich, has been 

 discovered in the Jurassic of Wyoming, North America, the contents 

 of which have been made known by Professor Marsh, while another 

 fertile source of these remains occurs in the Laramie beds of the 

 Upper Cretaceous of the United States. 1 



jNl The whole of the Mesozoic mammals at present known may be 

 divided into two great groups, the one characterised by a type of 

 dentition more or less clearly resembling that found among the 

 existing Polyprotodont Marsupials, while the other presents an 

 altogether peculiar modification, recalling in some respects that of 

 the Diprotodont Marsupials, although differing so decidedly as to 



Fig. 24. —Frontal and oral aspects of tlie cranium of Tritylodon longcevus ; from tlie Karoo 

 system of Basuto-land, South Africa. § natural size. (After Owen.) 



show that the owners of this form of dentition cannot be included 

 in that group. 



MuUituberculata. — The name Multituberculata has been proposed 

 for the group exhibiting the type of dentition last mentioned, and 

 is generally adopted, although the term Allotheria has been also 

 suggested. The essential characteristic of the dentition of this group 

 is the presence of a single scalpriform incisor on each side of the 



1 The subjects referred to under this heading are mostly described and figured 

 in detail in Owen's "Monograph of the Fossil Mammalia of the Mesozoic Forma- 

 tions," Palxontographical Society's Publications, 1871 ; and in various papers by 

 Marsh, in the American Journal of Science and Arts, 1878-89. Important con- 

 tributions to our knowledge of these forms have also been made by Professors Cope 

 and Osborn, and the reader should especially consult the memoir by the latter 

 writer on the "Structure and Affinities of the Mesozoic Mammals," published in 

 the Journal of the Philadelphia Academy (1888), vol. ix. 



