512 PALEONTOLOGY OF ILLINOIS. 



though abundant, proved very deceptive, and it was only after 

 much search that zoaria sufficiently complete to show tlie plans 

 were obtained. After this point was determined it became clear 

 that the genus was divided into two unequal groups. In the 

 smaller one, containing only G. plumosa and G. sagenella, with 

 its two varieties, the zoarium forms an irregular bifoliate ex- 

 pansion, having both surfaces divided into larger or smaller 

 cup-shaped cavities by the bifurcation and coalescing of "the 

 sharp-edged high ridges. These ridges are composed of two 

 layers of zo(Bcia grown together back to back. The other group 

 comprises the balance of the species, excepting, G. nrichelinitt, 

 which is intermediate, its zoarium being primitively unilaminar 

 as in this group, while the upper surface of the expansion is 

 precisely like that in the first section. In G. keyserlingi and 

 the other species the zoarium differs from tha.t of G. sagenella 

 in having the ridges developed to such an extent that they 

 form large bifoliate expansions. These diverge from a small 

 basal expansion, covered on the lower side by a strongly 

 wrinkled epitheca. 



The identification of Prout's species was another difficult task, 

 but, at last, all were known with the exception of his Coscinium 

 wortheni. No species is known to me that is strictly identical 

 with the figure given in Vol. 2 of these reports. I am, how- 

 ever, strongly inclined to regard the species as founded upon 

 the basal portion of G. keyserlingi. An examination of the 

 original specimens alone can determine the point beyond dis- 

 pute. 



GLXTTOPOKA PLUMOSA Prout. 



PI. LXXVIH, fig. 3-3c. 



Coscinium plumosum Prout, 1860, Trans. St. Louis Acad. Sci. vol. I, p. 572. 

 Coscinium plumo sum Prout, 1866. Geol. Surv. III. vol. II, p. 414. PI. XXII, fig. 3, 3k. 



Zoarium consisting of one or more bilaminar thin expansions, 

 arising from a common attached base, each having both sur- 

 faces traversed by strong bifurcating and coalescing ridges, so 

 as to enclose large but rather shallow' concave spaces or cups, 

 varying so far as observed, in length from 20 to 30 mm., and 



