LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

CRETACEOUS 8EDQTENTAEY DIVISIONS 73
with which we are concerned represent migrations from opposite directions,
and in part from antipodal regions of origin, from which they have found
their ways into the emb&yments. The boreal character of the aucellan
element in the fauna of the southern district was recognized by Hyatt,
Smith, and Stanton. With this element, certain cephalopod genera
scantily represented in the Cottonwood district may have come by the
same route from the north during Paskenta (Valanginian) time. The
nearest analogues of these aucellan species are found in Russia, and some
of the associated cephalopods can be traced to the same source, or in the
same direction, namely, the bemBsellida, oleostephnnids, and Bochianilea.
As has been noted, the cephalopod faunas in the northern district, especially
those of the Horsetown group, are largely if not wholly of southern
or subtropical aspect. Their nearest relatives or allies are found in the
Cretaceous areas of southwestern Asia (Australia, Cutch, and Caucasus
Mountains), and they seem to have reached the eastern Pacific and this
embayment by way of a Southerly route, not yet fully understood, but
which is perhaps explainable in terms of paleogeographic conditions of
land and sea.
At first thought it has seemed surprising that the boreal fauna, represented
by species of A-uceUa, are found almost wholly south of the delta
area, whereas the subtropical faunas are so greatly developed in the
Cottonwood district north of the delta. The varied cephalopod faunas
of the Horsetown group in this area have been found very little represented
outside of it. These faunas include many genera and species of Phylloceras,
Lytcceras, Crioceras, Ancyloceras, Parahoplites, Acanihoplites, Sonneraiia,
Cleoniceraa, Beudaniiceras, Dc&noceras, and Puzosia, described
in the following pages. Their sources and migration routes muBt be
sought in southern, rather than in northern, latitudes.
That the contrasted faunas did not mingle more freely in the intervening
area is no doubt attributable to its deltaic character and to the dislike
most of the molluscan types had for the freshened, sediment-laden waters
of this area. This view does not wholly explain the presence of the boreal
fauna south of the delta and that of the southern fauna3 on its northern
flank. However, should the contrasted faunas prove to be not strictly
contemporary, which seems unlikely, thiB would not invalidate the views
here expressed but, in effect, might even support them. If we assume the
existence of two or more inlets from the sea to the south of Klamathonia,
and circulating currents within the embayment, the molluscan migrations
from the north along the then existing littoral would enter the embayment
at the first suitable gateway and from it would coloni2e the basin only
insofar as conditions permitted. If they were able to tolerate cool, fresh
and sometimes turbid water, they might travel northward along the west
border of the embayment for a considerable distance, until conditions