524 PAl^ONTOLOGY OF ILLINOIS. 



stome or lip upon the lower margin, arranged in longitudinal 

 and diagonally intersecting lines, the longitudinal arrangement 

 most prominent. Apertures six longitudinally, seven and a 

 half diagonally in two mm., from 0.16 to 0.2 mm. in longitu- 

 dinal diameter. Zocecial tube* at first prostrate, then bending 

 outward proceed to the surface at an angle of 45; provided 

 with two or three diaphragms. Interzooecial spaces at first oc- 

 cupied by a vesicular structure which is soon afterwards re- 

 placed by solid calcareous tissue, pierced by minute tubuli; the 

 latter are arranged very closely in a flexuous line, and at the 

 surface appear as minute granules occuping the ridges between 

 the longitudinal series of cells; also found in the end spaces. 



This species is intermediate in its characters between P. firm a 

 and P. gigantea. From both it differs in the obliquity of the 

 cell apertures, from the former in the absence of monticules or 

 distinct maculae, in having longer and somewhat larger cells, 

 and the median tubuli arranged in regular flexuous lines, from 

 the latter in having much thinner walled zooecia, and somewhat 

 different grow 7 th. 



Position and locality: Cincinnati group; very abundant at 

 Wilmington, 111. 



PACHYDICTYA GIGANTEA Ulrich. 



PI. XXXI, fig. 3-3e. 



Zoarium a very large undulated frond, the type specimen 

 which is not complete, being 10.5 cm. in height, 5 cm. in its 

 greatest width, and about 3.0 mm. in thickness. Surface with 

 maculae, arranged in more or less regular series, sometimes 

 broad and low, usually level with the general surface, occasion- 

 ally a little depressed, about three mm. apart. Apertures 

 arranged with great regularity in longitudinal and diagonal 

 series, approximately oval, surrounded by a slightly elevated 

 peristome, from 0.15 to 0.25 mm. in their longest diameter, 

 those immediately surrounding the maculae being the largest 

 with about eight in three mm. longitudinally. Zooecial tubes 

 but little prostrate at their origin, curving outwards in their 

 course to the surface, where they open with nearly direct aper- 



