LATER MESOZOIC INVERTEBRATE FAUNAS 419 
In my opinion direct connection has not been proved. In time range 
the Chico fauna apparently began somewhat earlier and continued 
somewhat later than the Colorado fauna of the interior sea but it did 
not extend to the end of the Cretaceous, and latest Cretaceous time 
is probably not represented by marine deposits on the Pacific coast. 
Colorado jauna.—On the Atlantic side of the continent and in the 
interior region the greatest marine invasion of Mesozoic time 
occurred after the close of the Comanche. ‘The sedimentation began 
with the Dakota sandstone but the first distinctive marine fauna 
is found in the overlying Benton shale of the Colorado group. The 
Colorado fauna as a whole is easily distinguished, although it is devel- 
oped in several distinct faunal zones and local facies. It is character- 
ized by Inoceramus labiatus and several other specific types of Inocera- 
mus, by certain forms of Scaphites, and by the keeled ammonites 
known as Prionotropis, Prionocyclus, and Mortoniceras, which are 
sometimes referred to Schloenbachia in the broad sense. The fauna 
has a very great distribution, extending from Mexico and Texas 
throughout the Great Plains and Rocky Mountain regions as far 
north as Peace River in Canada. It is considered probable, though 
the faunal evidence is too meager for positive assertion, that there 
was marine connection entirely through from the northern interior 
to the Arctic Ocean. No marine faunas of Colorado age are known 
in the Atlantic and Gulf borders east of western Arkansas, unless 
possibly the imperfectly known fauna of the Eutaw or “’Tombigbee”’ 
sand of Mississippi belongs to its latest phase. If the Colorado sea 
covered that area its deposits have been overlapped by later beds. 
The earliest marine fauna, that of the Magothy or “Cliffwood,” in 
New Jersey, is apparently later than Colorado. 
In the eastern and southeastern parts of the Colorado sea where the 
later Colorado deposits are calcareous, constituting the Austin chalk 
and the Niobrara formation, the fauna of these beds is different in 
character from that of the underlying Benton shale, and the Austin 
fauna is much larger and more varied than that of the Niobrara 
though their correlation is fixed by a sufficient number of identical 
species. The calcareous Niobrara is characteristically developed 
east of the mountains in Colorado and Kansas, and northward to 
the Black Hills and Manitoba, but farther west and northwest the 
