362 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



*J. Surface of Hallopora multitabulata Ulrich, X12, showing ornamented zooecial clo- 

 sures. 



10. Vertical section, X12, of a discoid bryozoan (Hesotrypa bifida Ulrich) from the 

 Middle Ordovician shales of Minnesota, showing curved diaphragms in the zooecia and 

 closely tabulated mesopores. 



11-12. Internal structure of Prasopont selwynt Nicholson, a small hemispherical Ordo- 

 vician bryozoan ; 11, a tangential section, x 12, exhibiting the zooecia and numerous 

 mesopores; 12, a vertical section, X12, showing the isolation and semicircular form of 

 the cystiphragms, the diaphragms, and the close tabulation of the mesopores. 



13. Vertical section, X6, of Hcmiphragma irvasum Ulrich from the Middle Ordovician 

 shales of Minnesota, a solid branching bryozoan characterized by the occurrence of 

 semidiaphragms in the mature region. 



14 15. Structure of a thin laminar bryozoan (Lioclema foliatum Ulrich) from the Lower 

 Carboniferous (Warsaw limestone) of Illinois; 14, tangential section, X18, exhibiting 

 numerous mesopores and the zooecia indented by large acanthopores ; 15, vertical sec- 

 tion, XlS, through the zoarium, showing a very short immature region and a develop- 

 ment of numerous mesopores and strong acanthopores in the mature zone. 



1G. Surface, X12, of a ramose species (Halloporina crenulata Ulrich) from the Middle 

 Ordovician shales of Minnesota, illustrating the occurrence at regular intervals of the 

 clusters of mesopores termed maculae. 



17-18. Thin sections of Stenopora, a typical Upper Paleozoic genus of the Trepostomata ; 

 17, vertical section, X 12, through the mature region of S. amwicana Ulrich, from the 

 Mississippian (Keokuk limestone) of Illinois. The beading of the walls and the oc- 

 currence of perforated diaphragms are characteristic features ; 18, tangential section, 

 X 12, showing wall .structure. 



19, 20. Two zooecia of Batostoma winohelli Ulrich, from Middle Ordovician shales of 

 Minnesota, X30, as seen in tangential sections (19) ; and a variety of the same species, 

 X30, in which the acanthopores are strongly developed and thin structure is ap- 

 parent (20). 



21. Surface, X12, of an incrusting bryozoan (AtactopereUa typicalis praecipta Ulrich) 

 from the Middle Ordovician shales of Minnesota, with a floriform aspect due to the inden- 

 tation of the zooecial cavities by numerous acanthopores. 



detailed classification of the Trepostomata may be found in the 

 Eastman edition of Zittel's Textbook of Paleontology, while most 

 of the genera and many species of the order are described and illus- 

 trated by Nicholson, 20 Ulrich, 21 Ulrich and Bassler, 22 Cumings, 23 

 and Bassler. 24 



Ulrich and Bassler, in their Revision of the Paleozoic Bryozoa, 

 1904, proposed two divisions of the Trepostomata based upon the 

 minute structure of the walls separating adjoining zooids. Of the 

 seven families now recognized under the Trepostomata, four have 

 the calcareous investment of adjoining zooecia amalgamated to- 

 gether so that one wall can not be distinguished from its neighbor. 

 In the remaining three families the walls retain their duplex char- 

 acter, and when the zooecia are adjacent their boundaries are 

 marked by a dark, divisional line. This line in all probability rep- 



20 1881. Nicholson, H. A. The Genus Monticulipora. 



21 1882-1884. Ulrich, E. O. American Paleozoic Bryozoa, Journal Cincinnati Society of 

 Natural History, vols. 5-7. 1890. Ulrich, E. O. Paleozoic Bryozoa, Geological Survey, 

 Illinois, vol. S. 1893. Ulrich, E. O. Lower Silurian Bryozoa, Geological Survey of 

 Minnesota, Final Report, vol. 3,, pt. 1. 



22 1904. Ulrich, E. O., and Bassler, R. S. Revision of the Paleozoic Bryozoa. Trepo- 

 stomata. Smithsonian Miscellaneous Collections, vol. 47, pp. 15—55. 



23 1908. Cumings, E. R. The Stratigraphy and Paleontology of the Ordovician Rocks 

 of Indiana. Thirty-second Annual Report, Department of Geology and Natural Resources 

 of Indiana, pp. 605-1190. 



24 1911. Bassler, R. S. Early Paleozoic Bryozoa of the Baltic Provinces. Bull. 77, U. S. 

 National Museum. 



