LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

84 LOWER CRETACEOUS DEPOSITS IN CALIFORNIA. AND OREGON
along the littoral zones westward icn these latitudes. Land animals would
of course require some epochs of complete land connection for their migrations,
but for marine animals it could be different. These may have
followed either the northern or southern borders of a land bridge between
the Asiatic and American continents. Whatever extensions of such land
may have existed toward the north is not known, but one may suppose
that a broad zone of land extended from Peru to Mexico, or even to southera
California, forming an effective barrier far west of the present Isthmus,
between the north Indo-Pacific and the Comanchean sea of Texas, Mexico,
and the Northern Andes. This isthmian land would explain the faunal
contrasts between the Shastan and Comanchean faunas already discussed,
and at the same time suggests routes of molluscan migrations between
opposite shores of the north Indo-Pacific ocean. Favorable currents
traversing the northern littoral of such land bridges during early and
middle Cretaceous times might bring the marine faunas of southwestern
Asiatic regions and of intervening stations into the American West Coast
embayments in which they are now found.
Diverse Sources.—In accord with the foregoing views, it may be suggested
that some elements in the early Cretaceous molluscan faunas of
the Sacramento embayment (e.g., Berriasellids) may have had progenitors
in the late Jurassic (Tithonian) stocks near at hand; other types (olcostephanids,
neocomitids) now found mingled with AuceUa may have come
into the embayment with them from northern European sources by way
of Arctic America; others (Spiliceraa, hoplitids, and crioceratids) may hove
come from southern or far western (Asiatic) sources by way of a south
Pacific corridor.
At the opening of Horsetown (Hauterivian) time a new order of life
began in the Great Valley trough, partly as the result of immigrations
from without, but in part from more local stocks. However, migrations
continued to arrive at intervals until the close of Horsetown time. In
the early part of this epoch generic types (Neocrcwpeditea, Lytoceras,
and Hopiocrioaeras) arose partly from local stocks. At later epochs
during Borremian time came Ancyloceras, Acrioceras, and Shaslicrioceras,
followed in early Aptian time by new lytoceratids, Parahoplitoides, Nautilus,
and forms of Acroleuthis. In early Gargaaian time came Awtraliccraw,
Tropacum, ShasUtceras, HemibacuUtes, and new forms of PhylUxxras,
soon followed by Parahoplitea, Ckefaniccros, and hamitids, and later by
Acantheplites, Doutnlleiceras, Sonneralia, Cleoniceras, and Beudanticcra*.
In brief, new assemblages continued to appear, bringing into the embayment
at successive intervals lineages of cephalopoda not here before,
although no doubt many were autochthonous. Meanwhile, many of the
earlier stocks and their descendants disappeared. Almost all the inhabitants
of Valanginian time had vanished before Barremian time or were