EAELY PALEOZOIC BEYOZOA OF THE BALTIC PROVINCES. 277 



generally entirely absent in the intermacular areas and occurring 

 only sparingly in the maculse. 



In vertical sections the tubes in the immature zone have thin, 

 somewhat flexuous walls and are crossed by very few diaphragms. 



Fio. 165.— Batostoma mickwitzi. a, tangential section, X20, shq-wtng wall structuee and size 

 OF zoceciAj ranging from the large ones of the macula to the smaller ordinary ones; 6, ver- 

 tical SECTION, X8, through BOTH REGIONS AND WITH A SECOND GROWTH OF ZOCECIA; C, SEVERAL TUBES 

 OF THE MATURE ZONE, X20, IN VERTICAL SECTION; d, VERTICAL SECTION, X20, SHOWING STRUCTURE OF 

 WALLS IN THE rMMATURE ZONE. WASSALEM BEDS (D3), UXNORM, ESTHONIA. 



The curve to the surface is gentle and the mature region is distin- 

 guished principally by the more abundant diaphragms and slightly 

 thicker walls. In the most crowded portion of the tubes three dia- 

 phragms occur in their own diameter, but one to two in this distance 

 is more often the rule. 



