EAELY PALEOZOIC BRYOZOA OP THE BALTIC PROVINCES. 215 



and the wall bearing it. Less pronounced examples of this sort are 

 seen in the preceding Stigmatella injlecta, in Orhipora indenta, and in 

 many other bryozoans not described in this volume. All gradations 

 may be traced from the extremely developed folds of the present 

 species to very simple indentations caused by the swelling of the 

 zooecial walls when an acanthopore is inserted; in fact, all such grada- 

 tions may be seen in a single specimen. Thus, in figure 118 c, this 

 portion of a tangential section passes through the basal part of a 

 mature region and shows no acanthopores at all in most of the zooecial 

 walls, and a few just beginning to indent the cavities of the tubes in 

 the upper third of the figure. Figure 118 6 exhibits the change from 

 the region of no acanthopores to a zone in which they develop the 

 four-sided Tetradium effect illustrated by Nicholson. 



The remaining figures, 118 d and 118 e, show the greatest develop- 

 ment of the infolded, acanthopore-bearing, zooecial wall that has 



Fig. 117.— Stigmatella fookdh. a, a large sPEcniEN of Callopora fooedh, of the natural size; 



b, TANGENTIAL SECTION OF THE SAME, SHOWING THE PSEUDOSEPTAL FOLDS IN THE AUTOPORES [ZOCECIA], 

 ENLARGED TWENTY TIMES; C, VERTICAL SECTION OF THE SAME, SIMILARLY ENLARGED, O, AUTOPOEES, 

 m, MESOPORES. OEDOVICLiN ROCKS OF ESTHONIA. (AFTEE NiCNOLSON.) 



been observed. In order to show the change most clearly, the zooecial 

 cavities in figure 118 & have been shaded. All of these figures are 

 drawn from the same tangential section which passes somewhat 

 obhquely through the zoarium with the result that one end cuts the 

 immature zone and the rest of the sectio.n passes through various 

 phases of the mature region. 



As a description of this species has never been given, the following 

 diagnosis is in order: 



Zoarium of small, depressed, hemispheric masses seldom over 20 

 mm. in diameter; basal side flat and covered with a concentrically 

 wrmkled epitheca, celluliferous surface smooth with clusters of larger 

 zooecia and more numerous mesopores at regular intervals. Zocecia 

 small, polygonal or petaloid, seven in 2 mm. with an average aper- 

 ture about 0.28 mm. in diameter. Mesopores angular, thin-walled, 

 and numerous enough to frequently surround the zooecia. Acantho- 

 pores very small, varying from two to eight to a zocecium and devel- 



