CRETACIC PERIOD 591 



columbella, Gryphcea newberryi, Gervillia propleura, Cardium pauper- 

 culum, Liopistha meeki, Glauconia coalvillensis, Pugnellus fusiformis, 

 Baculites gracilis, Metoicoceras swallovi, Scaphites warreni, and the keeled 

 ammonites Prionotropis, Prionocyclus, and Mortoniceras. The Colorado 

 is also marked by the absence of Heteroceras, Ptyclwceras, Anisomyon, 

 large Baculites, and the broad compressed forms of Inoceramus, as I. 

 sagensis and I. vanuxemi. 255 



The Mobrara chalk is typically developed only in eastern Colorado and 

 Kansas, and thence northward into Manitoba. To the south it connects 

 faunally through identical species with the Austin chalk of Texas, though 

 in the latter formation the fauna is a larger and more varied one. To 

 the west and northwest the Niobrara changes into shale^ and it is then 

 indistinguishable from the Benton, its fauna being here made up of Nio- 

 brara and Austin forms, Benton derivatives, and local species. 



Montana series, western. — The Colorado series is followed without 

 break by the Montana. The faunas of this series vary locally from typi- 

 cally marine to brackish and fresh-water deposits. The marine life is a 

 continuation of that of the Colorado with the addition of Arctic and 

 southern Atlantic migrants. Some of the more distinctive fossils are: 

 Anisomyon borealis, Inoceramus sagensis, I. sublwvis, I. crispii, I. tenui- 

 lineatus, I. vanuxemi, Ostrea pellucida, Veniella humilis, Gervillia sub- 

 tortuosa, Callista deweyi, Breviarca exigua, Mactra gracilis, Corbulamella 

 gregaria, Amauropsis paludino3formis, Anchura nebrascensis, Fasciolaria 

 cretacea, Scaphites conradi, S. nodosus, Placenticeras intercalare, P. ivliit- 

 fieldi, Baculites ovatus, B. compressus, B. grandis, Heteroceras, and Pty- 

 choceras. 



The lower part of the Montana series or, rather, the Pierre formation 

 (Claggett and equivalents) is nearly everywhere typically marine, but in 

 the western area of the Coloradoan sea, from central New Mexico into 

 southern Athabasca, there is more or less alternation of coal-bearing 

 brackish and fresh-water beds with local marine horizons. These in 

 part are the Judith Eiver-Belly Eiver beds and the Mesaverde formation, 

 unknown in the eastern area of the Coloradoan sea. Above this series, in 

 the Bearpaw and equivalent formations,, the marine deposits are again 

 more widespread in the Montana. The invertebrates "fall into three gen- 

 eral categories of marine, brackish-water, and fresh-water forms, the latter 

 including a few more or less doubtful land shells." The brackish-water 

 fauna .contains Ostrea, Mytilus, Modiola, Anomia, Corbicula, Panopcea, 



255 Stanton : Bull. no. 106, TJ. S. Geological Survey, 1893, and Journal of Geology, 

 1909, p. 419. 



