LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

82 3WJWEH CRETACEOUS DEPOSITS IN CALIFORNIA AND OREGON
were better known, our problem might be simplified, but on the other
hand the facts herein offered should aid in the study of this history itself.
It may be possible that during late Mesozoic time the eastern borders of
the Pacific basin were very different from those of the present and that
they may have afforded suitable environmental conditions for the development
of immigrant faunas and for their migrations to other shores, but of
this little con now be said. No important Jurassic faunas are known on
the West Coast from which the faunas of the Cretaceous, particularly
their varied cephalopoda, could have been derived. With regard to the
aucellan elements, similar difficulties are apparent. Although the boreal
aspect of both the late Jurassic (Knoxville) and the early Cretaceous
species of Aucella in the California and Oregon embaymenta is well known,
the types found in the two series are as distinct here as in Russia, where
the analogues of both are well known and occur in the same order. One
may accept for both a line of exchange through Arctic America, Alaska,
and British Columbia. Although a part of the Lower Cretaceous ammonoids
of the Oregon and California troughs have generic allies in the
upper Knoxville (Tithonian) deposits, including Phyttaceras, Lytoceras,
Berriasella, and Spiticeras, they are specifically too unlike to warrant a
belief of lineal descent, and certainly the more characteristic ammonoids
of the Paskenta group cannot be traced to local Tithonian sources. Most
of them have closer allies in more distant regions. The belcmnoids of
the two series are of different types, as they are in other regions, and the
same is true for other classes of invertebrates. Some of the hoplitids
(Neocomites and Thurmannia), olcostephanids, and crioceratids may have
come with A uceUa from northern Europe and Arctic America. In general,
however, the cephalopoda of the Paskenta group in Oregon and California
seem to have come from sources within the environs of the greater
Pacific basin. An unknown but considerable number seems to have come
from northern latitudes, but many more must have reached these embayments
from more southern sources, or by way of them.
In contrast with the few groups of cephalopods found in the older
Cretaceous is the great number in the Horsetown group, as known in the
Cottonwood district. The more characteristic genera found in these
beds (Hauterivian to Aptian) are not known from any other West Coast
areas, north or south of the Great Valley trough. For the great majority
of them, sources must be sought in subtropical regions, from which the
routes of migration are now obscured, probably owing to the tectonic
history of the eastern border of the Pacific basin. Close relationships
are to be seen in many of the Horsetown ammonoids and contemporary
cephalopods in southwestern Asia (Spiti, Caucasus, Cutch, and Australia),
particularly in the Batremian and Aptian genera of the Parahoplitidae,
lytoceratids, ancyloceratids, and various others. Some of the