LATER MESOZOIC INVERTEBRATE FAUNAS 191 



In my opinion direct connection has not been proved. In time range 

 the Chico fauna apparently began somewhat eariier 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 fauna. — 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 



