EAELY PALEOZOIC BEYOZOA OF THE BALTIC PROVINCES. 



303 



closed at the surface. Acanthopores in the usual two sets, but in old 

 examples there is little difference in their respective size and they 

 are so numerous that a granulose appearance is imparted to the 

 zoarial surface. An ordinary zooecium is 0.33 mm. in diameter 

 with five in 2 mm. 



The important features of thin sections have been indicated in the 

 remarks on the genus. A tangential section of a young example is 

 shown in figure 185 a, mainly to exhibit the two sets of acanthopores, 

 the larger set in this case being located about the zooecia in pairs, 

 and the smaller in the walls of the mesopores forming a macula. 

 Tangential sections of an older example show numerous acantho- 

 pores of more equal size. 



Vertical sections show the usual close tabulation in the mesopores, 



Fig. 185.— Dittopoka clav^foemis. o, tangential section, x20, of a young example SHOwma 



TWO sets OF ACANTHOPORES AND SHAPE OF ZOCECIA DISTINCTLY: 6, VERTICAL SECTION OF THE SAME SPECI- 

 MEN, X20, ILLUSTEATENG DISTRIBUTION OF SEMIDLAPHRAGMS. GLAUCONITE LIMESTONE (B2), EEVAL, 

 ESTHONIA. 



but the zooecial tubes have well-marked semidiaphragms in both the 

 immature and mature regions. The outer region of the mesopores 

 is usually filled with a dense tissue through which the acanthopores 

 penetrate. 



The only bryozoan with a similar method of growth is the Stiqma- 

 tella claviformis (Ulrich) from the Wassalem beds, but its zooecial 

 structure is so different that comparisons are unnecessary. The 

 associated Dittopora annulata has a similar internal structure, but its 

 growth is strictly ramose. 



Occurrence. — Common in the Glauconite limestone (B2) at Pulkowa, 

 Wassilkowa, Tswos, and other localities in the government of St. 

 Petersburg; in the same formation at Reval and at Strietburg, in 

 Esthonia. 



Plesiotypes.—Cfit. Nos. 57390 to 57396, U.S.N.M. 



British Museum, specimens from Reval, Esthonia. 



