THE MISSISSIPPIAN PERIOD 615 







forms disappeared for reasons quite unknown. In the Osage 

 and Genevieve faunas, echinoids for the first time showed signs of 

 the prominent development they were to attain later. 



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

 or by other conditions, for they were more numerous than in the 

 Osage fauna. A compound coral (Lithostrolion canadense) became 

 conspicuous in the St. Louis limestone at some localities. Aside 

 from this, the simple horn-shaped forms remained 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. 433) , 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 

 whose slight resemblance to Archimedes' screw gave it name. 

 Archimedes became so abundant in the Kaskaskia epoch that a 

 part of the series is known as the Archimedes limestone, on 

 account of the great abundance of the fossils of this genus. 



For the first time there is clear evidence that the protozoans 

 were an important factor in the fauna, though it is not to be under- 

 stood that they were not really plentiful before; but they are here 

 recorded in certain limestones (p. 598) of the St. Louis series, some 

 layers of which are almost wholly composed of foraminiferal shells 

 of one species (Endoihym baileyi, Fig. 433). 



A notable modification took place in the braehiopods (Fig. 433) , 

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

 teristic. Many large spirifers disappeared, though small ones 

 remained. An odd feature was the diminutive size of the 

 brachiopods in the Bedford limestone of Indiana at Spergen Hill 

 and elsewhere. The associated fossils of other kinds were also 

 dwarfed, implying pauperizing conditions of some sort, for the 

 spjecies seem to be identical with those that grew larger elsewhere. 

 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. 



