Progress in Mammalian Palaeontology. — Osborn. 225 
cause it now appears certain that Marsh's Pliohippus was 
an Upper M'iocene and not a Pliocene animal, and was, 
moreover, apparently on a side line not leading directly into 
Equus (Gidley, Fig. 4). Thus not only is the Pliocene 
plains fauna sparsely known but the Pliocene forest fauna 
is wholly unknown. 
The Pleistocene Fauna. 
Equivalent to (1) the Preglacial, Forest Beds of Nor- 
folk (St. Prest. Durfort, Malbattu, Peyrolles), (2) Glacial, 
(Mid-Pleistocene, Lower Mid-Pleistocene), (3) Postglacial 
deposits of Northern Europe and Asia. 
Here again American palaeontology is far behind that 
of Europe as to knowledge of the chronological succession 
of deposits, and a vast amount of work remains to be done 
in the discrimination of geological and faunal stages, in the 
comparison of Eastern and Western cave — and sand — de- 
posits, and in the coordination of the first appearance of 
man with that of the mammalian succession. 
The advent of the true Equus marks the base of our 
Pleistocene, as shown in the sand deposits of the Western 
plains in the so-called Equus beds. The geographical dis- 
tribution and remarkable adaptive variation of the Pleisto- 
cene horses have now been fully worked out (Gidley 3 ), prov- 
ing that there are ten species characteristic of different 
localities, and ranging in size from E. giganteus larger than 
any modern horse, to the diminutive E. monte^umae. But 
nowhere in North America have horses been found contem- 
poraneous with man. 
Two chief advances have been made, first, the distinc- 
tion of plains and river, from forest faunas ; second, the 
exploration of two very remarkable cave deposits. 
The Western plains fauna of the Equus beds or Lower 
Pleistocene (Matthew 1 ) contains among the Carnivora, 
Canis, Dinocyon, Pelts; among the Rodentia, Fiber, Arvi, 
cola. Cynomys, 27iomomys, Castoroides; among the Eden- 
tata, Mylodon; among the Perissodactyla, three species of 
3 Tooth Characters and Revision of the Genus Equus. Bull. Amer. 
Mus. Nat. Hist., vol. iv, 1901, pp. 91-142. 
1 List of Pleistocene Fauna from Hay Springs, Nebr. Bull. Amer. 
Mus. Nat. Hist., yol. xvi, 1902, pp. 317-322. 
