32 



BULLETI]Sr 77, UNITED STATES NATIONAL MUSEUM. 



Hemiphragma ottawense (Foord). 

 Di2ylotrypa limitaris (Ulricli). 

 Diplotrypa neglecta Ulrich. 

 Monotrypa intabulata Ulricli. 



*Dianulites petropolitana Dybowski ( Mon- 

 otrypa cumulata Ulricli). 

 *Constellaria varia Ulricli. 



Tabulating the preceding lists of bryozoans with the total number of 

 American species and the number of species common to American 

 and Baltic areas, it will be noted, as may be determined from the 

 foUowing table, that on an average about 35 per cent of the species 

 are alike in the two regions. 



Summary of American and Baltic Ordovician bryozoa. 



Number of 

 species 

 _ . iiiiiiiuci yji common to 



r ormation . \ -r,-, p-,,i p^, y-, American 



and Baltic 



areas. 



Trenton group: 



Fusispii'a and Nematopora beds 



Clitambonites bed 



Black River gi-oup: 

 Decorab. shale — 



Pbylloporina bed 



Ctenodonta bed 



Rhinidictya bed 



Stietoporella bed 



Platteville limestone 



This percentage of common species is almost duplicated in the 

 summary of Baltic and American forms given on page 50, where the 

 species are tabulated with reference to the Russian formations. 

 Excluding the Lyckliolm and Borkholm bryozoans believed to be of 

 early Silurian age, there are 134 Ordovician species described in this 

 work, with 52 common to the two areas, thus giving practically the 

 same percentage. This similarity of percentages leads me to believe 

 that, even with more work upon these bryozoan faunas, the number 

 of new species will be counterbalanced by the discovery of American 

 forms in such a ratio as to maintain about this same figure. 



ORDOVICIAN AND EARLY SILURIAN STRATA OF ARCTIC AMERICA. 



The northward extension of the early Paleozoic strata in America 

 is of importance to substantiate the assumption that the Baltic 

 faunas usually entered this country from the north. As far back as 

 1854 siliceous limestones holding Black River or Chazy fossils were 

 reported by Alexander Murray as occurring on islands in Lake Nipis- 

 sing, northern Ontario. Later N. H. Winchell ^ described this lime- 

 stone as resting unconformably upon the gneiss and trap rocks of the 

 islands, and published the following list of fossils determined by 

 E. O. Ulrich: 



1 Amer. Geologist, vol. 18, 1896, pp. 178-179. 



