A REVISION OF THE PALEOZOIC BRYOZOA 

 By E. O. ULRICH and R. S. BASSLER 



Part I. — On Genera and Species of Ctenostomata 



In 1897 Mr. Ulrich undertook the preparation of as complete 

 a collection as possible of American fossil Bryozoa for the British 

 Museum of Natural History. At the same time the late Professor 

 Zittel of Munich wished a similar although somewhat smaller collec- 

 tion for his university. The junior author was at that time assisting 

 Mr, Ulrich, so the two decided, while preparing these collections, 

 to revise especially the generic and specific classification of the 

 Paleozoic Bryozoa, as thoroughly as the material at hand would 

 permit. With the exception of two short trips into the field, Mr. 

 Ulrich spent two years in the preparation and study of thin sections 

 and the separation of large collections embracing representative 

 examples of the whole class. Mr. Bassler, besides aiding Mr. 

 Ulrich in this work, also undertook the review of all the literature 

 of Paleozoic Bryozoa and the compilation of a card catalogue of 

 the American forms, using as a basis a list and bibliography pre- 

 pared some years before by Mr. Ulrich. In 1900, Mr. Bassler, 

 collaborating with Mr. John M. Nickles, who had prepared a 

 similar card catalogue, published^ much of the information con- 

 cerning genera and all of the synonymy learned during the prepara- 

 tion of these collections and catalogues. 



During the course of our studies, particularly of the Trepostomata, 

 some four or five thousand thin sections were prepared. Of many 

 species we made sometimes as many as a dozen sets of sections to 

 determine the specific variation. Naturally the immense quantity 

 of material studied afforded many new species, and these, besides 

 aiding the strict characterization of the previously established generic 

 groups, also served in distinguishing certain new genera that pre- 

 viously had been known only from species more or less imperfectly 

 understood and therefore difficult to classify satisfactorily. At the 

 same time a number of wholly new genera were determined, while 

 other, perhaps equally distinct, groups require further material to 

 prove the permanence of their peculiarities. 



'^Bulletin 173, U. S. Geological Survey. 



256 



