EAELY PALEOZOIC BRYOZOA OF THE BALTIC PROVINCES. 315 



although the distribution of diaphragms is not so regular. In the 

 immature zone these structures are horizontal and are placed from 

 two to a single tube diameter apart. The mature zones are recog- 

 nized almost solely by the crowding of the diaphragms, three or four 

 of which occur in the space of one diameter. Here, too, they are often 

 curved; indeed, in some tubes a few cystiphragms are introduced. 

 This crowding and the curved diaphragms are well shown in the 

 mature zone of Nicholson's vertical section (fig. 192 d), while an 

 occurrence of cystiphragms is illustrated in figure 193 d. The usual 



Fig. 194.— DIPLOTETPA PETKOPOLITANA. a, small portion of a TANGENTLiL SECTION, X8; 6, PARTS 0¥ 

 SAME SECTION, X20; C, VERTICAL SECTION, X8, WITH TWO MATtJEE ZONES BOUNDING AN IMMATURE REGION; 

 d, PORTION OF SAME, X20, SHOWING THE BEADED WALLS AND ALSO TWO MESOPORES REPLACED BY A 

 ZOOEQUM. LTCKHOLM LIMESTONE (Fl), KeRTEL, ISLAND OF DaGO, ESTHONIA. 



distribution of diaphragms for the species is shown in figure 194, 

 where another peculiarity of the species as well as the genus is illus- 

 trated, namely, the change of a mesopore into a zooecium. 



As pointed out by Ulrich,^ Diplotrypa petropolitana is closely alhed 

 to D. westoni Ulrich from the Trenton formation of Manitoba, Canada, 

 but the few diaphragms of the latter will distinguish it. The large 

 zooecia and rather crowded tabulation distinguish D. petropolitaTia 

 from all described members of the genus. Compared with associated 



1 Contr. Micro-Pal. Cambro-Sil., pt. 2, 1889, p. 30. 

 92602'— Bull. 77— 11 22 



