EAELY PALEOZOIC BEYOZOA OF THE BALTIC PROVINCES. 319 



colony. The oldest portion of a zoarium the base and the edge of 

 the cellulif erous surface, exhibits a dense, sohd surface to the unaided 

 eye, but when magnified, this region is seen to be made up of minute, 

 thick-walled, round to hexagonal cells of a uniform diameter. In 



Fig. 198.— Diploteypa bicoenis. a, tangential section, x20, showing the shape, size, and arrange- 

 ment OF ZOCECIA AND MESOPOEES EST THE MATURE ZONE. THE MESOPORES AEE SHADED FOR CONVEN- 

 IENCE OF recognition; 6, part of the same SECTION, X40; C, VERTICAL SECTION, X20, SHOWING THE 

 REGULAR AND CLOSE ARRANGEMENT OF TABULiE IN THE MESOPORES AISTD THEIR lEEEGULAR DISTRIBU- 

 TION IN THE ZOCECIA; d, VERTICAL SECTION, X40, ILLUSTRATING THE NOEMAL DISTRIBUTION OF DLA.- 

 PHRAGMS IN THE TWO KINDS OF TUBES. HERE ONE OF THE ZOCECIA REPLACES TWO MESOPORES. THIN 

 SECTIONS OF THE BASAL PART OF THIS SAME ZOARIUM ARE SHOWN IN FIG. 197. JEWE LIMESTONE (Dl), 



Baron Toll's estate, Esthonia. 



tangential sections through this and the adjacent normal celluhf erous 

 area, the zocEcial walls are seen to become so thickened by the deposit 

 of a dense tissue that the diameter of the orifice is the same as that 

 of the mesopores. The same thickening of the walls is well exhibited in 

 vertical sections where the mesopores also are noted to be invested 



