334 PALAEONTOLOGY OF ILLINOIS. 



have arranged under three suborders: CYCLOSTOMATA, TREPO- 

 STOMATA and CRYPTOSTOMATA, while the CTENOSTOMATA and 

 CHEILOSTOMATA also seem to have had a meagre representation. 



I. Ctenostomata. These Bryozoa as they are known in the 

 recent state, are characterized by a horny or membranous test; 

 on which account it is generally supposed that they have not 

 been capable of preservation by fossilization. One of their lead- 

 ing characteristics is the curious opercular mechanism of the 

 zooecium. The upper portion of the cell wall is thinner than the 

 rest, and terminates above in a number of delicate setae held 

 together by a thin transparent membrane. When the polypide 

 retreats the whole of this anterior portion is drawn in, and the 

 setae, being brought close together, form an operculum-like cover- 

 ing above the tentacles. The zoarium is often creeping, with 

 the flask-shaped zooecia arranged in single series resembling 

 Hippothoa and Stomatopora. Frequently the zooecia stand erect 

 upon a cylindrical stem, while in other cases they are immersed. 



As already stated the horny membranous test is strongly 

 against finding fossil remains of CTENOSTOMATA, still there are 

 two Palaeozoic genera that agree so nearly in habit with certain 

 recent members of the suborder that I can not do otherwise 

 than place them together. These genera are Ascodictyon Nich. 

 and Eth. jr., and Rhopalonaria* The genus Ascodictyon was 

 established by the authors cited for the reception of certain 

 problematic fossils which occur usually as radially arranged, 

 extremely delicate fibres on shells, etc. The fibres vary in length 

 and are sometimes bulbous or inflated at intervals. In a few 

 cases, too, the fossil consists of small clusters of bulbous vesicles 

 which are arranged in a radial manner. When more than one 

 cluster is preserved they seem to be connected to each other by 

 a thread-like fibre. The substance of the fossil appears to be 

 corneo-calcareous. Ascodictyon may be compared with species 

 of Valkeria, Bowerbankia and Cylindrcecium, as figured by 

 Hincks (British Marine Polyzoa). The Silurian genus Rhopalo- 

 imitates Hippothoa in its zoarial habit. The zooecia are 



* Since the above was written I have learned that in the Ann. & Mag. of Nat. Hist 

 for Aug., 1884, Mr. Vine in discussing these two genera, suggests a comparison with the 

 stoloniferous VESICULABID^: (CTENOSTOMATA). 



