356 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



6. Rhopalonaria rcnosa Ulrich. Early Silurian of Southwestern Ohio. Portions of the 

 encrusting colony, X12. 



7-8. Vinella radiciformis confcrta Ulrich from the Silurian (Waldron) shales of In- 

 diana; (7) the encrusting colony, X3, showing the close development of the nuclei; 

 (8) nuclei and their connecting stolons, X 12. 



9. Rhopalonaria attonuaia Ulrich and Bassler, X6, Silurian (Rochester) shales of New 

 York. This characteristic Silurian species occurs as an excavated mold on shells and 

 other fossils. 



10. Allonema fusiformis Nicholson and Etheridge, jr. Middle Devonian shale of Michi- 

 gan. Portion of the encrusting colony, X6, composed of numerous vesicles. 



11. Ascodictyon stcllatum Nicholson and Etheridge, jr. Middle Devonian shales of 

 Western New York. A cluster, X 12, composed of vesicles one of which shows the 

 punctate structure. 



12. The excavation on the surface of a shell left by a species of Terebripora, X 10, from 

 the Miocene rocks of North Carolina. 



13-14. Cylindroccium dilatatum Hincks. (13) Incrusting basal part of this recent 

 species, X 12, .showing spinose dilatations at the base of the zooecia ; (14) the erect 

 zooecia, X12, attached to the tubular basal expansion. 



15. Avenella fusca Dalyell, a living species, XlO, illustrating the tubular stolons with 

 the erect zooecia. 



16. Arachnidium hippo thoides Hincks. Part of the encrusting network of zooecia, XlO, 

 connected by slender fibers. 



17-18. Bowerbankia pustulosa Ellis and Solander. (17) The erect zoarium, two-thirds 

 natural size, showing the group of polypides at regular intervals; (18) a group, XlO, 

 with the polypides expanded. (Figs. 13-18, after Hincks.) 



Among the living Ctenostomata Alcyonidium and related genera 

 grow into soft incrustations or into masses 6 inches or more high in 

 which the zooecia are closely united. In Bowerbankia the erect 

 branching zoarium bears tufts of zooecia at regular intervals, while 

 in Amathia an interesting spiral arrangement of the branches occurs. 

 The Ctenostomata are typically marine, but a few genera have a 

 tendency to live in estuaries. For this reason and other character- 

 istics they are believed to have given origin to the exclusively fresh- 

 water Phylactolaemata, 



ORDER 2. GTCLOSTOMATA. 



The zooecia in this order are simple calcareous tubes, usually with- 

 out transverse partitions, with a plain, rounded, uncontracted orifice 

 not closed by an operculum. The walls are thin and minutely porous 

 and do not show, the complicated structures visible in the Cheilo- 

 stomata or Trepostomata. The ovicell, when present, is an enlarged 

 zooecium or an inflation of the zoarial surface. The zoarium assumes 

 very many different forms of growth (pi. 3), although the method of 

 growth is quite constant for a species. 



Hitherto the families and genera of Cyclostomata have been 

 founded almost entirely upon the form of the zoarium and the ar- 

 rangement of the zooecia. As a result, very complicated artificial 

 classifications have been proposed, which the reader may consult in 

 the review given by Gregory in 1909. 16 



16 1909. Gregory. Catalogue of Fossil Bryozoa in Department of Geology, British Mu- 

 seum, Cretaceous, vol. 2, pp. xxiv-xli. 



