314 PALEONTOLOGY OF ILLINOIS. 



exhibit the same condition precisely when they have undergone 

 some weathering. With this new light upon the subject, Steno- 

 pora once again reverts to the imperforate forms, since we have 

 no conclusive evidence showing that any true species of the genus 

 possessed perforate walls. 



That some at least of the structures mentioned in the section 

 devoted to "median tubuli" aided in the inter-communication 

 of the zocecia, I cannot doubt, but nothing comparable to the 

 communication plates of the CHEILOSTOMATA has yet been recog- 

 nized in their Palaeozoic ancestors. As only the "immature" or 

 prostrate region of the zocecia of Ptilodictya, and other genera 

 of that type, is equivalent to the ordinary Cheilostomatous 

 cell, the communication plates, if they existed at all, ought to 

 be found in the thin walls that separate the zooecia in that 

 region. 



VII. Opercular structures, diaphragms and cystiphragms. 

 Opercular structures seem to have been present in most Bryo- 

 zoa, yet among fossil forms they are of extremely rare occur- 

 rence. Among the CHEILOSTOMATA their absence is readily ac- 

 counted for by the fact that the operculum in these is incapa- 

 ble of preservation, since it is composed of horny substance. In 

 the CRYPTOSTOMATA, which are Palaeozoic representatives of the 

 CHEILOSTOMATA, the operculum was most probably also horny. 

 In many cases, however, notably the FENESTELLID.E, the second- 

 ary or superficial aperture was closed by a thin centrally per- 

 forated plate, which, most probably was not developed till at 

 the final period of growth. This seems to be shown by the 

 absence of diaphragms in the tubular orifice beneath it, and the 

 fact that the plate has been observed in fully matured examples 

 only. This plate has also been detected in tangential sections 

 of very old examples in which the zocecial orifices are covered 

 with a dense layer of sclerenchyma having the same structure 

 as the dense substance of the non-poriferous side of the branches. 

 In these sections the central perforation in the operculear plate 

 is surrounded by numerous minute dark spots, which probably 

 represent former pores. Among the CYCLOSTOMATA the zocecial 

 tube is similarly closed by a perforated frail calcareous plate 

 situated usually a short distance from the aperture. This plate 



