EAllLY PALEOZOIC BRYOZOA OP THE BALTIC PROVINCES. 3 



America was made possible. My studies on these collections have 

 been in progress for some years, but other duties, as well as the large 

 amount of labor required to study and illustrate the faunas, have 

 prevented prompt publication. 



During the progress of these studies I have had the continued 

 advice of my friends. Dr. E. O. Ulrich, of the United States Geological 

 Survey, and Prof. Charles Schuchert, of Yale University, to both of 

 whom I am under obligations. As many of the Russian forms have 

 proved to be identical with American species described by Dr. Ulrich, 

 it has been my especial good fortune to enjoy the benefit of his mature 

 judgment upon difficult and debatable points. Prof. Schuchert has 

 kindly read and criticized that part of my manuscript dealing with 

 the geology and stratigraphy. I am also indebted to Miss Fran- 

 cisca Wieser, who, in her usual skilled manner, has prepared many 

 of the drawings and has retouched all of the photographs illustrating 

 this volume. The illustrations of the internal structure were drawn 

 under a camera lucida by myself. 



In order that a representative collection of these bryozoan faunas 

 should be available to European students, the United States National 

 Museum has forwarded an almost complete set of the species herein 

 described to the British Museum. On the other hand, the authorities 

 of the British Museum have deposited a good set of Ordovician bryo- 

 zoans from the island of Oeland in the collections of the United States 

 National Museum. As a result of this exchange of material, the 

 collections of both institutions have practically a complete representa- 

 tive set of these species. A second quite complete set has been 

 placed in the Mickwitz collection, now preserved in the museum at 

 Reval. 



DISTRIBUTION OF EARLY PALEOZOIC BRYOZOA. 



The exceeding richness of the North American Ordovician strata 

 in Bryozoa has long been known, and " monticuliporoids " from the 

 Cincinnati and other regions usually form a part of every paleonto- 

 logical collection. At the present time about 500 species of Bryozoa 

 have been described from American strata of this geologic period, 

 and an equally large number of new forms is known. With further 

 searching it is almost inevitable that many more new species will be 

 discovered — indeed, a conservative estimate would place the described 

 species at not more than one- third of the probable number of Ordo- 

 vician forms. The oldest known American form is a new species of 

 Nicholsonella occurring in the Beekmantown rocks of Arkansas. 

 Exclusive of this occurrence, the class is unrepresented in America 

 until the Stones River division of the Mohawkian is reached, when a 

 prolific fauna is encountered. 



