THE CRETACEOUS PERIOD 771 



istic aspects to the fauna, while corals and crinoids, so long asso- 

 ciated with clear seas, were not abundant. 



In the -clastic formations, pelecypods and gastropods are abundant 

 and characteristic fossils (Fig. 522) . It will be seen by a glance at 

 the figures that they have a modern appearance. Cephalopods 

 were still abundant, though ammonites were in their decline and 

 were showing erratic divergencies of form, attended by excessive 

 ornamentation, comparable to that which marked corresponding 

 stages of the trilobites and crinoids. Odd forms of partial un- 

 coiling, or of spiral and other unusual forms of coiling, were common 

 (Fig. 523). Interesting forms, perhaps to be classed here, were 

 the Baculites (i), which resumed the straight form of the primitive 

 Orthoceras, while retaining the very complicated sutures of the 

 Ammonites (c). 



Map work. Folios of the U. S. Geological Survey containing good maps 

 for the study of the Comanchean and Cretaceous systems are the following: 

 Arizona, Bisbee, Clifton, Globe; California, Coif ax, Lassens Peak, Mother 

 Lode, Redding, Sacramento, San Luis; Colorado, Anthracite and Crested Butte, 

 Elmoro, La Plata, Nepesta, Pueblo, Spanish Peaks, Telluride, Walsenburg; 

 District of Columbia, Washington; Delaware-Maryland-New Jersey, Dover; 

 Oklahoma, Atoka, Tishomingo; Montana, Fort Benton, Little Belt, Livings- 

 ton, Three Forks; New York, New York City (Staten Island and Harlem 

 sheets); Oregon, Roseburg, Coos Bay, Port Orford; South Dakota, Edgemont, 

 Oelrichs; Texas, Austin, Nueces, Uvalde; Virginia, Fredericksburg; Wyoming, 

 Alladin, Cloud Peak-Fort McKinney, Devils Tower, Hartville, Newcastle, 

 Sundance, Yellowstone. 



Both Comanchean and Cretaceous are classed as Cretaceous in the folios, 

 though often distinguished in the text and on the maps. 



