18G 
ON THE STRUCTURE AND CLASSIFICATION 
ON THE STRUCTURE AND CLASSIFICATION OF THE MESOZOIC MAMMALIA. 
liY Henry Fairfield Osborn. 
In 1871, when Profes.sor Owen completed his “ Monograph upon the Fossil Mam- 
malia of the Mesozoic Formations,” there were but twenty genera of this period known 
to science, two of which only, Dromaiherium and Miorolestes, were from Triassic beds, 
and tlie remainder from the Jurassic. The genera have now been increased to over 
thirty-five, five of these coming from the Trias. These numbers alone cause us to 
modify our former ideas as to the paucity of mammalian life in the Mesozoic period. 
In a rapid survey of this ancient fauna, we are at first struck with the very great 
diversity which prevails in the form and arrangement of the teeth, consisting of six or 
seven wholly distinct types ; and this at a zoological period which we have been 
accustomed to consider as the dawn of mammalian life. The above types, although 
jirimitivc, are essentially mammalian. In one genus only, Dromaiherium, do we find 
clear evidence of reptilian affinity in the dentition. Then we are surprised to discover 
a very close zoological relationship betiveen fossil faunae of the same age, but having 
a wide gi'ographical distribution. The most striking instance of this is the parallelism 
iM'twemi the American and British upper Jurassic fauna. For, among the thirteen 
gencni discoveri'd by Professor Marsh in the Atlantosaurus Beds, or American Upper 
.Bira.ssic, six have their counterparts in the English Middle Purbcck, and the family 
characters arc very close as regards the remainder. The two American triassic 
gmicra, Dromaiherium and Microconodem, are isolated, but the genus Witylodon, from 
the South .\frican Irisis, has a close ally in Triglyphus from the Rhaetic beds near 
Stuttgart, as Ncumayr has pointed out ; it is also related to the genus Bolodon from 
the 1 urhock, and probably lias a lateral successor in Polymastodon, a highly modified 
form found in the .\merican Puerco beds. The-inost remarkable distribution, both 
geographically and stratigraphicaUy, has, however, been enjoyed by Plagiaulax, which 
extends froin Microlesies, in the Trias of Germany and England, to Ptilodus in the 
I ucrco of New Mexico, and Neoplagiaulax in the lower Eocene of France, probably 
terminating, by a side branch, in Thylacoleo of the Australian Quaternary. The fol- 
lowing table shows the geological and geographical distribution of the known meso- 
zoic genera and of their tertiary descendants: 
