OF THE MESOZOIC MAMMALIA. 
257 
The grouping of all these genera in one distinct order is, however, impracticable ; 
first, because the members of at least one family present distinctively marsupial char- 
acters ; second, it is impossible, with our present knowledge, to assign a single 
character of ordinal value which is universal ; third, as to the minor question of sys- 
tematic arrangement, there is no precedent for including in one order, such types as 
Kurtodon, Stylacodon and Triconodon, in which the teeth are as diverse as in the 
recent Rodentia, Insectivora and Carnivora. If distinct from the Marsupialia, the 
mesozoic mammals certainly represent an equivalent subdivision of the Metatheria. 
Of the nine characters assigned to the Pantotlieria by Marsh, only two rest upon 
actual observation through the entire series, viz. : the mylohyoid groove and the unan- 
kylosed symphysis. The latter is not distinctive. The mylohyoid groove, is shown 
by data collected in the Appendix, to have little taxonomic value. The character, (1) 
“ cerebral hemispheres smooth,” was undoubtedly true of all mammals of this period, 
and can be actually observed in one of the unique Yale College specimens. Each of 
the remaining characters excludes one and, in some cases, several genera : (2) Teeth 
exceeding, or equaling, the normal number, 44. (3) Premolars and molars imper- 
fectly differentiated. (4) Canine teeth with bifid or grooved fangs. (7) Angle of lower 
jaw without distinct inflection. (8) Angle of jaw near or below horizon of teeth. 
(9) Condyle vertical or round, not transverse.^ 
The supposition that all these mammals can be placed in the Marsupialia is 
equally untenable, or, at least, it may be said to rest upon no foundation whatever. 
It has' been the fate of numerous primitive mammals, at the period of their discovery, 
to be placed without much reason or question in this order. The Greodonta is a con- 
spicuous instance. This tendency is a remnant of the old doctrine that all primitive 
mammals were Marsupials, which is opposed on numerous grounds by the more 
recent view that the Marsupials and Placentals were branches from a common steal ^ ; 
in fact the peculiar reduction and succession of the teeth® and mode of placentation 
exclude the derivation of the Placentals from the Marsupials, and we now have 
abundant evidence that these eccentricities of the marsupial dentition were fully 
developed as early as the later Mesozoic period. Does this not indicate that the 
separation of these two stocks had already taken place 1 Where are we to look for 
1 (2) Excludes Paurodon. (3) In all the genera known to the writer, the premolars, where present, are 
well differentiated from the molars. (4) Excludes Phascolotlierium and Amblotherium. (7) Excludes Triconodon 
in some of its species, T. ferox. (8) Excludes Amblotherium, Stylodon and Amphitylus. (9) Excludes Phasco- 
lotherium and Triconodon. 
* See Huxley, “ On the Arrangement of the Mammalia,” Proc. Zool. Soc., Dec. 14, 1880, p. 649. The 
systematic portion of this valuable article was partly anticipated by Gill, in his “ Arrangement of the Fami- 
lies and Sub-Families of Mammals,” Smithson. Contrib.. Vol. XI, 1874. Following Gill’s line of suggestion, 
Huxley proposed the term Prototheria for the representatives of this stem stage. 
3 Oldfield Thomas, in his recent memoir, shows that the consideration of the reduction and succession 
of the teeth alone forces us to the same conclusion. 
