438 MISSISSIPPIAN PERIOD 



crinoids, however, did not show so remarkable a decline, and new and 

 curious forms appeared. Blastoids had their climax here so far as 

 numbers of individuals are concerned, although there was greater 

 diversity in the Osage fauna. A swift decline seems to have fol- 

 lowed this climax, and the beautiful forms disappeared for reasons 

 quite unknown. 



Polyps seem to have profited by the decline of the crinoids, or 

 for other reasons, for they were more numerous than in the Osage 

 fauna. The simple horn-shaped forms were the most common. 

 Bryozoans made a new departure in their mode of support. The 

 delicate branches of their colonies could not extend themselves 

 indefinitely without special means of support. As one mode of 

 securing this support, the genus Archimedes (Fig. 381, b), which made 

 its first appearance in the Osage, secreted an axis with a spiral 

 flange upon which the colony spread itself, producing a unique form 

 resembling slightly Archimedes' screw. Archimedes became so 

 abundant in the Kaskaskia epoch that a part of the series is known 

 as the Archimedes limestone, because of the great abundance of 

 fossils of this genus. 



A notable change took place in the brachiopods (Fig. 381), 

 though Productus (g and ti) continued to be abundant and charac- 

 teristic. An odd feature was the small size of the brachiopods in the 

 Bedford limestone of Indiana. The associated fossils of other kinds 

 also were dwarfed, implying pauperizing conditions of some sort, 

 for the species seem to be identical with those that grew larger else- 

 where. It is not improbable that this limestone was deposited in a 

 partially isolated body of water that was so highly charged with 

 lime and other salts as to be somewhat unfavorable to life. A 

 similar dwarfed fauna is recorded from Idaho. 



Among mollusks,pelecypods (Fig. 381, i,j) were rather abundant, 

 and some of them still had a Devonian aspect. Those ii} the Indi- 

 ana foraminiferal limestone were small, like the brachiopods. 

 Gastropods were more diversified than in the Osage fauna, and some 

 Devonian genera which apparently had been absent from the Osage, 

 reappeared. Sharks (Fig. 382) were important and other fish were 

 present. 



The most striking peculiarity of the fauna resulted from the 

 invasion of the more conservative fauna of Devonian aspect from 

 the sea of the Great Basin, and perhaps from a similar incursion of 

 lingering forms from the Waverly gulf on the east. The remarkable 



