306 PALEONTOLOGY OF ILLINOIS. 



walls in a transverse direction, is a point not yet determined. 

 The evidence at hand favors the last possibility more than the 

 first 



Sections oiRhomboporalepidodendroides, E. crassa, Bactropoi-a, 

 simplex, Stenopora? signata and several other species, exhibit a 

 most peculiar character, which is perhaps best shown in the 

 species last mentioned. In tangential sections of these forms, 

 the portion af the wall immediately surrounding the zocecial 

 cavity displays one or two series of minute spots with a dark 

 or lucid central space, while each ''acanthopore" is likewise en- 

 circled by a row of the same spots, which again in the central 

 region of the walls are arranged is closely approximating clus- 

 ters of six or more. As viewed in vertical sections, they still 

 present only the appearance of minute spots, and I have not 

 observed anything showing that they communicated either with 

 each other or the zooecia, nor can I offer any explanation of 

 the singular structures- 



In a large proportion (perhaps all) of the FENESTELLID/E the 

 non-poriferous side of the branches as well as the spaces between 

 the zooecia apertures are pierced (?) by very minute vertical 

 tubes, which compare with the surface pores of both the CYCLOS- 

 TOMATA and CHEILOSTOMATA. 



VI. Structure of the walls of the zocecia. From a zoological 

 point of view the minute structure of the cell walls is of the 

 first importance. Unfortunately, however, the process of fossili- 

 zation has so often obliterated the finer details, that great 

 caution and careful examination are necessary if we would 

 obtain a correct idea of their structure. Still, with the aid of 

 good sections the differentiations are determinable and may be 

 utilized with great advantage in the framing of our classifica- 

 tion. Before we proceed to discuss the more common modifica- 

 tions in the minute structure of the zooecial walls, it is necessary 

 to notice a very important feature of zoarial anatomy. 



a. "Mature" and ''immature 7 ' regions. In the TREPOSTO- 

 MATA and CRYPTOSTOMATA, namely, the tubular zooecia which 

 make up the zoarium become more or less modified in their 

 structure as they approach the surface and pass into their final 

 stage of growth. In all cases, whatever their structure may be 



