348 PALEONTOLOGY OF ILLINOIS. 



ous features of tangential sections, and, when the same is suffi- 

 ciently well preserved, of the surface. The superficial aperture 

 is sometimes circular, but usually of a broad elliptical form, 

 and surrounded by a narrow sloping area, wanting in Pachy- 

 dictyae, where it is replaced by a faint peristome. 



The family as now restricted comprises the following six 

 genera: Stictopora Hall, Eurydictya, Dicranopora, Pachy- 

 dictya, Phyllodictya and Euspilopora Ulrich. The first three 

 are distinguished by zoarial differences, the remainder by slight 

 zocecial modifications as well. I find that Pachydictya ap- 

 proaches the CYSTODICTYONID^E in having vesicular tissue, the 

 anterior end of the primitive cell sometimes rounded, and a 

 slight peristome around the superficial aperture. Opposed to 

 these points of resemblance is the absence of the lunarium, the 

 presence of median tubuli, and the fact that the prevailing 

 form of the primitive cell is often either sub-quadrate or hexa- 

 gonal. The chain of evidence showing the relation of the genus 

 to Stictopora* and Eurydictya is completely established through 

 the intermediate species Pachydictya splendens and P. firm a 

 Compared with the PTILODICTYONID.E we find a general agree- 

 ment in the arrangement of parts, but a closer examination 

 will reveal important differences. Of these the absence of 

 median tubuli among the genera of that family, alone need be 

 mentioned here. 



(3) PTILODICTYONID.E: Bifoliate zoaria, continuous or jointed, 

 forming leaf-like expansions or compressed branching stems; 

 thus having the same range of zoarial characters as the CYSTO- 

 DICTYONID^E and STICTOPORHLE. The outline of the primitive cell 

 is oblong hexagonal or quadrate, or rhomboidal. In the typical 

 genera both the inferior and superior hemisepta are well devel- 

 oped, but in other divisions either one or both are apparently 

 wanting. In old examples of Ptilodictya they may be repre- 

 sented at successive levels, so as to simulate diaphragms (see 

 fig. 6 and 11). Vestibular portion of zooecia with thick walls, 

 the central region between the cavities being usually also of 

 conspicuously lighter or darker color than the rest. In typical 

 forms this region is transversely lined and margined on each 



*Kegarding Stictopora fenestrata as the type of the genus. If, however, 8. ele- 

 gantula is insisted upon as the type, then;the reader will please read lihinidictya Ulrich, 

 where I have written Ktictopora Hall. 



