604 TA (Og (CISVANI IED BIRILION. 
type. Such faunas appear to characterize the Middle Silurian, 
the Middle Ordovician, the Carboniferous, and the Cretaceous 
periods, and in a less pronounced degree the Devonian, the 
Jurassic, and the early Tertiary. 
(2) Conditions imposing general restrictional evolution of marine 
life.—\f{ at the close of a period of great base leveling attended 
by expansional evolution of marine life, as just outlined, an 
epoch of profound readjustment to the earth’s accumulated 
contractional stresses ensues, the great feature of which consists 
of the sinking of the ocean basins or scme large part of them, 
the effect is to withdraw the waters from the surface of the con- 
tinental platforms into the basins thus increased in capacity 
and to establish a new shore line somewhere near the edge of 
the continental platforms. If the enlargement of the capacities 
of the ocean basins is pronounced, a new shore line may be 
established, not upon the upper face of the continental platforms, 
but upon their abysmal slopes. In this case the shallow-water belt 
will be narrow and will consist of a rapidly shelving shore tract. 
It is obvious that the great expansional fauna which has occupied 
the broad sea-shelves and the extended epicontinental seas of 
the preceding period will be compelled to follow the retiring sea 
and crowd itself into this restricted zone on the abysmal slope 
of the continents. It is further obvious that, in addition to the 
restricted area into which the fauna is thus forced, the new con- 
ditions will be in many respects uncongenial, for the streams 
will be rejuvenated and the amount of land wash will be greatly 
increased. Those species whose existence is dependent upon 
clear seas will be in imminent danger of extinction. Certain 
species to which these conditions are congenial may on the 
other hand be favored, but the grand result must necessarily be 
the destruction of the larger part of the previous expansional 
faunaand the forced adaptation of the remainder to new, and on 
the whole sterile and hostile conditions. A stage of general 
repressional evolution is thereby inaugurated and, in a compara- 
tively short period, it is safe to assume, all or nearly all pre- 
ceding species will have passed out of existence and new species, 
