THE MISSISSIPPIAN PERIOD. 531 



New devices of the bryozoans. — The reticulate bryozoans made 

 a new departure in their mode of support. The delicate branches 

 of their colonies cannot extend themselves indefinitely without special 

 means of support. As one mode of securing this support, the genus 

 Lyropora secreted a U-shaped calcareous base, upon the inner con- 

 cave surface of which the rest of the colony spread itself. As another 

 and more successful mode, the genus Archimedes (Fig. 238, q), 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 has given it name. 

 The Archimedes became so abundant in the Kaskaskia epoch that 

 some beds of the Kaskaskia formations are known as the Archimedes 

 limestone, or account of the great abundance of the axes of this genus. 



The protozoans make a record. — For the first time there is clear 

 evidence that the protozoans were an important factor in the fauna. 

 It is not to be understood that they were not really a vital element 

 before, for theoretically there is good reason to believe that they had 

 long been abundant; but they now record themselves in a so-called 

 oolitic limestone in the St. Louis formation, certain layers of which 

 are almost wholly composed of a foraminiferal shell, Endothyra baileyi 

 (Fig. 238, x and y). On account of their globular form, the shells of 

 this genus have been mistaken for oolitic concretions, and have given 

 a misleading name to the formation. 



Changes in the brachiopods. — A notable modification in the aspect 

 of the brachiopods took place (Fig. 238, g-p). The large spirifers 

 of the striatas type disappeared, though some small spirifers remained 

 (Fig. 238, /), while Productus (Fig. 238, g and h) continued to be very 

 abundant and characteristic. Seminula (Fig. 238, 1), Eumetria (Fig. 238, 

 m), and Spiriferina (Fig. 238, k) were well represented. An odd fea- 

 ture was the diminutive size of the brachiopods, as well as of all other 

 forms, in the Spergen Hill fauna of the Bedford limestone of Indiana, 

 although they were remarkably abundant. The associated fossils 

 of other kinds were also diminutive, implying general pauperizing 

 conditions of some sort, for the species 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 diminutive fauna is recorded from Idaho. 



