360 GEOLOGICAL BIOLOGY. 



perhaps no more remarkable fact is established in the history 

 of organisms than the sudden expansion of the placental mam- 

 mals in the Eocene. 



Over fifty genera, representing the chief ordinal types of 

 the placental mammals, are already reported from the lowest 

 Eocene, none having been discovered in the underlying Creta- 

 ceous. In Europe alone Zittel reports for the fauna of the 

 Upper Eocene about iio genera and about 200 species. To 

 show the richness of this fauna, in spite of the imperfection 

 of the records, he cites the facts that " Our present European 

 land mammalian fauna contains 54 genera with about 150 

 species, and of these 60 per cent belong to the microfauna, 

 consisting of the smaller forms of Rodents, Insectivora, Bats, 

 and Carnivora, for which the conditions of preservation in 

 earlier epochs were very unfavorable" (" The Geological De- 

 velopment, Descent, and Distribution of the Mammalia," by 

 Karl A. von Zittel, Geol. Mag., Dec, ill., vol. X., Sept., 

 Oct., Nov., 1893). 



If we glance at the whole group of mammals, we find the 

 actually known forms included in three subclasses : the (I) 

 Prototheria, with the order Monotremata; (II) Metatheria, 

 represented by the order Marsupialia; and (III) Eutheria, or 

 the Placentalia. 



There can be no doubt as to the higher rank of the Pla- 

 centalia over the marsupial and monotreme types. No cer- 

 tain traces of the Placentalia are known to occur below the 

 Eocene. Stegadon, a genus of the Tillodontia, is thought 

 to have appeared possibly in earlier beds. 



Of these mammals, ten orders, fossil and recent, are recog- 

 nized. Two of these are marine — Sirenia and Cetacea. 

 The Edentata is a South American order, and has its repre- 

 sentatives in the earliest known South American mammalian 

 fauna (Vera Cruz fauna of Patagonia), which is probably 

 equivalent to the northern Eocene. 



If we omit the above three orders, of the remaining seven 

 orders of land mammalia five are represented in the older 

 Eocene of Europe — the Ungulates, with 5 suborders; the 

 Rodents, the Insectivores, the (Carnivora) Creodonta, the 

 Prosimise — the forerunners of, if not true. Primates. 



