41-4 PALEONTOLOGY OF ILLINOIS. 



Coscinium plumosum, Prout. 



PL 22, fig. 3, 3 a. 

 Coscinium plumosum, Pkout, 18C0. Proceed. St. Louis Acad. Sci., vol. i, p. 572. 



Polyzoum an irregular, leaf-like expansion, midrib sharp, 

 crust-like, raised irregularly above the surface, more or less 

 alternately branching, the spaces between the branches being 

 more or less concave, and rising upon their outer borders to be 

 united with other concave expansions from other branches. 

 Where the expansions are large, the long, narrow, rythmical 

 dimples, and corresponding ridges between them, are sometimes 

 curved like long plumes, gracefully waving over one another, 

 but the distribution of the dimples is mostly irregular, from 

 the irregular growth of the polyzoum. In one specimen, be- 

 longing to my own cabinet, the opposite face of the polyzoum 

 is broken up into cup-shaped cavities about the size and shape 

 of those which characterize the Michelinia favosa, Koninck. 



These cup-shaped cells are bounded by interstitial plates, 

 with thin, salient and entire lips, denuded of chalices near the 

 margin, and are marked by rythmical and abortive spaces, 

 occasionally towards the centre. Sometimes this cupuliferous 

 expansion is found separate from the wider and more plumose 

 form of the polyzoum, and would, by those who are not careful 

 in distinguishing forms, be most probably referred to Michelinia. 



Geological position and locality : Lower beds of the St. Louis group; War- 

 saw, Illinois, and Barrett's Station, St. Louis county, Missouri. 



Coscinium Michelinia, Prout. 



Coscinium Michelinia, Prout, 1860. Proceed. St. Louis Acad. Sci., vol. i, p. 573. 



Polyzoum encrusting, extending over a considerable surface, 

 divided into larger or smaller, more or less hexagonal, cup- 



