EARLY PALEOZOIC BRYOZOA OF THE BALTIC PROVINCES. 13 



holding less glauconite than either of the two preceding zones. As 

 the list shows, its fauna is closely related to the preceding as well as 

 the following strata. 



Lamansky's subdivisions of B2 and B3 were published after the 

 bryozoans studied were collected, so that I am unable to place the 

 species in their proper zones. 



B3. Orthoceras limestone. — This is a hard gray limestone varying 

 from 3 to 20 feet in thickness and crowded with individuals belonging 

 to several species of Orthoceras and Vaginoceras. The names Ortho- 

 ceratite, Orthoceras, Vaginoceras, or Vaginaten limestone have at 

 various times been applied to these rocks. Of these, I have selected 

 the name Orthoceras limestone for use in the present work. The 

 phosphatic concretionary layer at the base of the Orthoceras lime- 

 stone has already been mentioned, as has also the sandy phase of the 

 formation west of Reval. The fauna of the limestone is large and 

 includes a considerable number of cephalopods and gastropods, both 

 of these groups being very sparsely represented in the underlying 

 strata. The trilobites, however, lead in the number of species, with 

 the gastropods second in importance. Among the trilobites are 

 species of AsapJius, Ampyx, Megalaspis, Pterygometopus, and Cyiele, as 

 well as many other genera. The gastropod genera Maclurea, Oxydis- 

 cus, Salpingostoma, RapMstoma, Bucania, Holopea, Clisiospira, and 

 Sinuites, well known in American deposits, are represented. The 

 brachiopods are more abundant than the gastropods in individuals 

 if not specifically, but the bryozoans are rather sparsely represented 

 and belong almost entirely to new species. Orthis, Clitamhonites, 

 Poramhonites, Plectamhonites, and Rajinesquina are, as usual in these 

 deposits, the most important of the brachiopod genera. 



The Orthoceras limestone has been studied carefully by Lamansky, 

 who recognizes three horizons, namely, B3a, the horizon holding 

 AsapTius expansus arnd A. lamansMi, B3b, with AsapTius raniceps as 

 the characteristic fossil, and B3c, with AsapTius eichwaldi and 

 Ptychopyge glohifrons as horizon markers. The fauna of each of these 

 divisions is indicated in the lists on pages 19 to 25. 



Following the Orthoceras limestone is another concretionary layer 

 known as the ''Ober Linsenschicht." As many new species and a 

 decided change in lithology occur above this layer, the Kussian geolo- 

 gists begin a new formation with the next horizon. 



CI. EcJiinospJierites limestone. — Formation C is divided into two 

 well-defined members, namely, Cl, a thin-bedded limestone named 

 after its characteristic fossil, a species of EcJiinospJierites, followed by 

 C2, a bituminous shale deriving its name from exposures at Kuckers, 

 Esthonia. A third member, C3, a hard limestone 20 to 30 feet thick, 

 with numerous siliceous concretions, has been distinguished at a few 

 localities as the Itfer limestone. The most characteristic fossils of 



