176 BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



in rocks most probably of Borkholm age, I have introduced the 

 species as a member of this fauna. As will be seen from the accom- 

 panying figure, F. striolata is quite a typic'al Fenestella, differing but 

 little from a number of middle Silurian forms which might be men- 

 tioned. Indeed, it is possible that Eichwald's figured type was 

 derived from younger rocks than the Borkholm limestone, since he 

 also registers the species from ''le calcaire sUiceux a Pentameres de 

 Talkhof en Livonie." However this may be, I feel reasonably cer- 

 tain that the genus Fenestella in Europe begins in the Lyckholm or 

 Borkholm limestone, and for the present will recognize F. striolata 

 as a Borkholm species. As the Borkholm fauna contains so many 

 species in common with the Richmond group of North America, 

 which holds a neat Fenestella described by Whitfield as F. granulosa, 

 it is possible that Fenestella striolata has more relationship with the 

 American form than at present supposed. 



Occurrence. — Silurian at Talkhof in Livonia, and at Borkholm, 

 Esthonia. 



Order TREPOSTOMATA Ulrieh. 



Trepostomatous Bryozoa usually form so large a part of the col- 

 lections from American Ordovician horizons that, in number of 

 specimens at least, species of this order seem to make up the greater 

 part of a fauna. This is equally true for the various Baltic Ordo- 

 vician formations, indeed, certain limestone layers, such as those in 

 the Wassalem beds (D3), are literally one mass of ramose and other 

 bryozoans. As will be seen from the following pages, a fairly large 

 fauna of Trepostomata has been determined, but I am confident that 

 careful collecting with the Bryozoa particularly in mind will bring to 

 light many more species. The Trepostomata, which include nearly 

 all of the forms classed frequently under the general term ''monti- 

 culiporoids," have been the occasion of some discussion with reference 

 to their systematic position. Without entering into detail at the 

 present time, I stUl believe that all of the evidence favors their posi- 

 tion in the classification as a weli-defmed order of Bryozoa rather 

 than an aberrant division of the Anthozoa. 



The division Trepostomata was established by Ulrieh as a suborder 

 to include the majority of the Paleozoic Bryozoa. The principal 

 peculiarity of the division, now ranked as an order, was the fact that 

 at a certain point in the course of the tubes to the surface they change 

 their character entirely. This change occurs at the beginning of the 

 peripheral or mature zone of the zoarium and consists of the develop- 

 ment of mesopores, acanthopores, more numerous diaplu-agms, 

 opercula, and similar structures of the more mature zooid. In the 

 axial or immature portion of the tubes the walls are simple, thin, 

 and prismatic, with diaphragms few or wanting. The occurrence of 



