4 BULLETIlSr 11, UNITED STATES NATIONAL MUSEUM. 



Fortunately for correlation purposes the described forms are 

 almost entirely from standard Ordovician sections in which the 

 position and range of each species is fairly well known. Thus, in 

 New York and Canada at least the characteristic bryozoans of the 

 various horizons have been determined; in the Cincinnati uplift a 

 large fauna has been described; in Illinois the faunas of formations, 

 either poorly represented or absent in the other regions, are partially 

 worked out, while, finally, the species of the Middle Ordovician in 

 Minnesota have been published in considerable detail. 



In marked contrast with this prolific representation is the exceed- 

 ing scarceness of Bryozoa in most of the Ordovician areas of Europe. 

 In the British Isles very few species have been noted, partly because 

 the strata here are mainly slates and sandstones, indicative of condi- 

 tions ill fitted for bryozoan life. Similar conditions obtain in the 

 Bohemian Ordovician, and correspondingly few bryozoans are 

 known. The Ordovician of Spain and France has likewise yielded 

 few specimens of this class. However, certain limestones in Scandi- 

 navia and most of the Ordovician and early Silurian formations of 

 western Russia abound in Bryozoa, and it is with the faunas of these 

 two areas that the present work is concerned. 



GENERAL GEOLOGY OF BALTIC RUSSIA. 



The lower Paleozoic formations of Russia outcrop in a broad belt 

 extending from Lake Ladoga westward to the islands of Oesel and 

 Dago, where they are hidden to a great extent by very recent deposits. 

 The extent of outcrop and the larger unconformities are indicated on 

 the accompanying map published by Dr. F. von Schmidt (PI. 1), 

 which is introduced to show the location of the various fossil localities 

 in addition to the geology. This belt of outcrop continues under the 

 Baltic Sea in a broad curve to the southwest, for essentially the same 

 formations of the lower part of the section are exposed on the island 

 of Oeland. The Borkholm limestone is known to outcrop under the 

 Baltic because of the numerous bowlders containing this fauna in the 

 drift on the island of Gothland. These drift bowlders have been 

 carefully studied by Wiman, who has published an excellent paper 

 upon the subject entitled "Tiber die Borkholmer Schicht im Mittel- 

 baltischen Silurgebiet." ^ The accompanying sketch map (fig. 1), 

 modified after a chart by von Schmidt, shows the distribution of 

 the Ordovician and SUurian rocks in this western portion of the 

 Baltic area. 



The Cambrian and Ordovician deposits of the area are of unusual 

 interest in that they are made up in considerable part of unconsoli- 



1 BuU. Geol. Inst. Univ. Upsala, vol. 5, pt. 2, No. 10, 1902, pp. 149-222, pis. 5-8. 



