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.! 
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 
Fic, 24.—Frontal and oral aspects of the cranium of Yritylodon longcevus; from the Karoo 
system of Basuto-land, South Africa, 4% natural size. (After Owen.) 
show that the owners of this form of dentition cannot be included 
in that group. 
Multituberculata.—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,” Paleontographical 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 hy 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. 
