SILURIAN FAUNA INTERPRETED EPICONTINENTALLY 693 
existence, and grew in extent as the period advanced. These 
were the Silurian seas, and in them there evolved a new assem- 
blage of shallow-water organisms, the Silurian fauna. This 
fauna was derived genetically from those remnants of the earlier 
Ordovician fauna which had happened to survive in favored 
localities, but with the very general expansion of the shallow 
waters, a great expansional evolution took place and many 
organic characteristics, showing a notable advance in differentia- 
tion beyond that of Ordovician time, came into existence and 
were characteristic of Silurian time.* 
As the Ordovician waters were gradually drawn off from the 
continental platform, the once broad sea, extending from east- 
ern Canada to beyond the present Rocky Mountains, with its 
wealth of organic life, was gradually contracted, and its life 
either gradually became extinct or was forced into modified 
forms or compelled to emigrate under unfavorable conditions. 
The remains of the last survivors, of this once magnificent 
fauna, within this area, are now found in the lower beds of the 
Medina formation ini Virginia.? With the passing of these last 
survivors, the interior Medina basin became a lifeless tract so 
far as any evidence has been left to us, save for some low types 
of aquatic plants and a few worm burrows. It was probably an 
isolated basin. 
With the encroachment of the Silurian sea upon the con- 
tinent, a junction was at last effected with the Medina basin, 
and again marine conditions, and a marine fauna occupied the 
area. 
The Medina fauna of New York,3 which has been described 
from the upper beds of the formation, signals the return of the 
marine conditions. This fauna is a meager one containing but 
thirteen species, most of which would not be out of place in 
either an Ordovician or a Silurian fauna, but the presence of a 
See “A Systematic source of Evolution of Provincial Faunas,” by T. C. CHAM- 
BERLIN, JoUR. GEOL., Vol. VI, p. 597. 
2 STEVENSON, Proc. Am. Phil. Soc., Vol. XXII, pp. 142 and 150; Vol. XXIV, 
pp. 85, 87 and 94. 
3 Pal. N. Y., Vol. II. 
