462 PALEONTOLOGY OF ILLINOIS. 



MONOTRYPA Nicholson, 1879. 



(PaL Tab. Corals, p. 293.) 

 (For generic diagnosis see page 379.) 



MONOTRYPA RECTIMTJRALIS Ulrich. 



PI. XXXVIII, Fig. 4-4b. 



Zoarium massive, varying from lenticular to sub-spherical in 

 shape, growing to be ten or more cm. in diameter. Surface 

 generally smooth, presenting, occasionally, faintly elevated 

 clusters of cell apertures, about one-half larger than the ordi- 

 nary size. Zooecia angular, direct, with exceedingly thin straight 

 walls; apertures polygonal, about six of the average size in the 

 space of two mm. Straight horizontal diaphragms from two to 

 four tube diameters apart. No acanthopores observed. There 

 are indications that the zoarium is built up of a number of 

 successive superposed mature regions with a very narrow imma- 

 ture region, separating two succeeding mature regions. 



The simplicity of its structure is the most marked feature of 

 this species. It differs from M. undulata, Nich., and M. subglo- 

 bosa Ulrich, in having straight instead of undulating or crenu- 

 lated walls. 



Position and locality: The type specimens are from the Cin- 

 cinnati group, in Alexander Co., 111. Examples apparently of 

 this species are quite common near Savannah, 111., and occur 

 more rarely at Cincinnati, Ohio. 



CERAMOPORA Hall, 1852. 



(Pal. N. Y. Vol. II, p. 168.) 

 (For generic diagnosis see page 380.) 



The type species of this genus is so peculiar in certain re- 

 spects that I have deemed it advisable to figure its internal 

 structure. This is done on PI. XXXIX, by fig. 1-16. A brief 

 description of the species is as follows. 



