INVERTEBRATES. 421 



Cyclopora polymorpha, Prout. 



PL 21, fig. 5, 5 a, 5 6. 

 Cyclopora polymorpha, Prout, 1860. Proceed. St. Louis Acad. ScL, vol. 1, p. 578. 



Polyzoum a large incrusting expansion, with the ridges vari- 

 ously contorted without the appearance of a disc, although it 

 seems to have had several centres of development. Chalices 

 larger than in the two former species, with the interchalicular 

 spaces more rugose ; chalices on both faces, with evidences 

 here and there of a disposition in the layers to superposition. 

 The specimen from which this description is drawn is about 

 four inches broad by as many long, and the whole expansion 

 of the polyzoum seems to have been several times as broad as 

 the space comprehended by these measurements. It is, we 

 think, evidently specifically different from the two preceding 

 forms. 



Geological position and locality : Chester group, Pope county, Illinois. 



Genus POLYPORA, McCoy, 1844. 



(Carb. Poss. Ireland, p. 206.) 



Polypora Halliana, Prout. 



PL 21, fig. 4, 4a, 46. 

 Polypora Halliana, Prout, 1860. Proceed. St. Louis Acad. ScL, vol. i, p. 580. 



Polyzoum a broad fan-shaped expansion, with apparently 

 entire rigid longitudinal rays radiating from a central point, 

 with fenestrules obvious to the naked eye. Longitudinal rays 

 regular, on a plane of expansion only slightly waved without 

 folds or plications, round, straight, or direct, bifurcating nearly 

 oppositely near the base, about two lines near the middle, 

 about four and not so frequently towards the border. Dissepi- 

 ments mostly alternate, small, depressed, often not much more 

 than connecting points between the longitudinal rays ; about 

 one-fourth as long as the fenestrules, and somewhat expanded 



