336 



BtTLLETIN 11, UNITED STATES NATIONAL MUSEUM. 



Hall, Dybowski figured a vertical section (see pi. 5, fig. 1 T), wliich is 

 undoubtedly of this form. His tangential section (figs. 1-1 e) may 

 also be from specimens of the same species. His figures 1 / to 1 Ti, 

 however, are undoubtedly based upon thin sections of Prasopora 

 simulatrix Ulrich, an abundant hemispheric bryozoan in the Trenton 

 strata of America. Dybowski erroneously identified these several 

 species with Callopora nummiformis Hall, a small, discoid bryozoan 

 from the Silurian rocks of New York and Indiana. The latter species, 

 wliich belongs to Mesotrypa, a genus of the Monticuliporidse, has been 

 figured by the present writer,^ and comparison will show the error of 

 Dybowski's identification. 



Fig. 211.— Hallopoea ? dtbowsioi. o, tangential section, X20, •with zocecu. less angulae than 



usual; 6, VEETICAL SECTION, X8, SHOWING CUEVED DIAPHEAGMS; C, SEVEEAL TUBES OF THE SAME, X20. 

 WESENBEEG LIMESTONE (E), WeSENBEEG, EsTHONL*.. 



So far as the zoarial characters go, the description of Diplotrypa 

 petropolitana will apply to the present form in every respect, and even 

 the internal structure is somewhat similar. Tangential sections of 

 the two species are almost identical. Slight differences exist, but 

 these can not be described as permanent because of the variation in 

 each in shape of zooecia and number of mesopores. In vertical sec- 

 tions the widely separated diaphragms, three to ten tube diameters 

 apart, in the zooecial tubes of HJ dyhowskii, are in marked contrast 

 with the numerous diaphragms of the similar species. That this is a 

 constant difference was shown by a study of over one hundred speci- 

 mens of both species, each specimen falling into its particular place, 

 with no intermediate forms. Continuing the comparison with Z>. 

 petropolitana it is found that H.f dylowsJcii does not possess the 

 beaded walls characteristic of Diplotrypa. Moreover, as in other 



1 BuU. 292, U, S. Geol. Surv., 1906, pi. 10, flgs. 1, 2. 



