522 PALAEONTOLOGY OF ILLINOIS. 



EUEYDICTYA STERLINGENSIS UMch. 

 PI. XXX, Fig. 2-2a. 



Zoarium an irregular frond, sometimes appearing to branch. 

 Surface smooth, destitute of maculae. Zooecia arranged in very 

 regular longitudinal and diagonally intersecting series. Aper- 

 tures round, about 0.12 mm. in diameter, six vertically and 

 seven diagonally in two mm.; separated by very wide inter- 

 spaces, about 0.20 mm. wide, which are filled with a great 

 number of minute granules arranged in several series about each 

 aperture. 



The form of growth of this species is like that of E. multipora 

 Hall, but that species has the interspaces very narrow, and 

 cells so arranged as to appear between elevated longitudinal 

 ridges, while the minute tubuli form a longitudinal line between 

 the adjoining vertical series of apertures and never occupy the 

 end interspaces. 



Position and locality: Cincinnati group, Sterling, 111. 



PACHYDICTYA Ulrich, 1882. 



(Jour. Gin. Soc. Nat. Hist. Vol. V, p, 152.) 

 (For generic diagnosis see page 390.) 



This genus proves to be one of the most important divisions 

 of the lower Palaeozoic Bryozoa. The three species that were 

 known to me when the genus was proposed, have since been re- 

 inforced by nineteen others, varying in time from the base of 

 the Trenton to the Lower Helderberg. The form of the zoarium 

 varies greatly in the different species, some being narrow rib- 

 bon-like, a few wider and stronger, while others again form 

 broad undulating expansions of considerable thickness. In all, 

 however, the minute structure is remarkably constant. The ex- 

 tremes in this respect so far noticed, are furnished by the five 

 species here described. The structure of P. robusta Ulr., the type 

 of the genus, is intermediate between that of P. everetti and 

 P. firma, while that of P. gig-ant ea, is very much like that of 

 P. acuta Hall, P. occidentalis Ulr., F. obesa Foerste, and sev- 

 eral other species. P. splendens suggests Eurydictya, but not 

 very strongly. 



