SLUDIES POR STUDENTS, 489 
some time in the life of the individual or cycle. This is often in 
the larval state, but it is known that certain species of brachiopods, 
which are swimmers only in the cephalula stage, are distributed 
over thousands of miles. It is an important fact that ammonites, 
which were free in the adult stage, and may have been swimmers, 
though most probably chiefly crawlers, are not distributed much 
more widely than lamellibranchs or others of the lower inverte- 
brates Ehusithe Upper Uriassic taunal of (California containe 
many species that are found in the Alps and the Himalayas, and 
among these there are several species of the Aviculide, which in 
the adult stage were attached by a byssus. The Lower 
Cretaceous Knoxville fauna of California contains along with 
ammonites a good representation of European lamellibranchs ; 
and the Horsetown and Chico faunas of California contain fully 
as many species of lower invertebrates as’ of ammonites that 
may with probability be referred to foreign species. It is clear, 
then, that their distribution has been due to faunal, rather than 
to individual migration. Faunal migration can only be possible 
under physical agencies acting slowly, and on a large scale. 
Marine currents along shore lines have been efficient aids to 
migration, not so much by transporting the young, as by trans- 
porting conditions suitable for life. 
Even with our currents continental margins offer ample oppor- 
tunities for faunal migration, for the crawling adults or the swim- 
ming young can find the constant conditions of depth, tempera- 
ture and food supply suitable for their life. 
Dr. W. H. Dall? has recently shown that tides on the archi- 
benthal continental margins have been influential in distributing 
faunas. But probably the great agent of distribution has been 
the slow shifting of shore lines, which not only aids but also 
compels migration. 
BARRIERS TO MIGRATION, 
Land barriers —Land masses present an insuperable barrier to 
all marine invertebrates; but if the bodies of land are short, and 
* Bull. Mus. Comp. Zodél., Vol. XII., No. 6, p. 179. 
