LATER MESOZOIC INVERTEBRATE FAUNAS 415 
EARLY CRETACEOUS FAUNAS 
At the beginning of the Cretaceous the two faunal provinces which 
have just been indicated were even more sharply defined than they 
had been in the Jurassic, and in each area the characteristic elements 
of the fauna are developed from the fauna that preceded it. The 
Shasta fauna on the one hand and the Comanche fauna on the other 
are always sharply contrasted, though each exhibited several facies. 
When compared with European faunas, one is in the beginning chiefly 
boreal and the other Mediterranean; one is associated with shales, 
sandstones, and conglomerates, the other, mainly with limestones. 
Shasta faunas.—The boreal Aucella fauna of the Knoxville for- 
mation is the earliest one in the Shasta series. It is distributed from 
the Arctic coast of Alaska to southern California but south of the 
Yukon never extending as far east as the late Jurassic fauna did. 
Cretaceous Aucella beds have been reported from Catorce, San Luis 
Potosi, Mexico, but it is probable that these are Jurassic on about 
the same horizon at which Aucella occurs at Mazapil. 
The succeeding Horsetown fauna though at first showing a transi- 
tion from the Knoxville fauna is, as a whole, remarkably distinct 
from it. It is characterized by the great abundance and variety of 
ammonites of types which in Europe are considered distinctive of 
the deeper water facies of the Mediterranean province. The boreal 
element is wanting, or at least inconspicuous. ‘This early Horsetown 
fauna is much less widely distributed than the Knoxville. In its 
typical development it is known in a relatively small-area in northern 
California and in Oregon. ‘Toward the close of the Horsetown the 
fauna was greatly modified by the introduction of many types that 
show relationship with the Cretaceous faunas of southern India and 
also with those of Japan. ‘This relationship was continued in the 
succeeding Upper Cretaceous faunas to such an extent that it is 
appropriate to speak of an Indo-Pacific province or region. This 
later Horsetown fauna was more widely distributed along the Pacific 
border and is well developed as far north as the Queen Charlotte 
Islands. 
The marked change at the close of the Knoxville when the fauna 
ceased to have a distinctively boreal character was probably due to a 
northern uplift which closed Bering Strait and other direct connections 
