EARLY PALEOZOIC BRYOZOA OF THE BALTIC PROVINCES. 



129 



aperture is about 0.10 mm. by 0.12 mm., and seven to eight may be 

 counted in 2 mm.; when well preseiwed, many of the apertures are 

 closed by a convex subcentrally perforated plate. Mesopores 

 small and sometimes few, but often numerous enough to isolate the 

 zooecia. Along the edges of the zoarial branches around the fenes- 

 trules, there is a band of mesopores 0.5 mm. or more in width. 



The internal features illustrated in figure 53, prepared from a Rus- 

 sian example, differ from typical species of StictoporeUa crihrosa only 

 in the scarcity of mesopores and the corresponding greater size of the 

 zooecia. In tangential sections the polygonal areas are plainly 

 marked out, and the cavity of the zooecium is seen to be subcircular or 

 ovate and of less diameter than the walls. The mesopores vary in 

 size and shape and are sometimes completely filled by deposits of 

 sclerenchyma. 



An examination of numerous specimens from both the American 

 and Russian localities shows that the zooecium has some variation in 

 size. This is due, as in 

 other bryozoans, to the 

 number of mesopores 

 present. When meso- 

 pores are few, the zooecia 

 are larger, but with nu- 

 merous mesopores the 

 zooecia become decreased 

 in size. In each case the 

 number of zooecia in a 

 given space is the same. 

 Ulrich's illustrations (fig. 

 52) show the condition in which mesopores are most numerous, while 

 the thin section shown in figure 53 illustrates the opposite condition. 



The occurrence of such highly organized and well-marked species 

 as Graptodictya proava (Eichwald) and Stictoporella crihrosa Ulrich 

 in both the American and Russian strata without a single variation 

 in their respective specific characters was one of the pleasant surprises 

 of my studies, and to me, at least, was good evidence of the great 

 stratigraphic value of these organisms. Moreover, these species 

 offer so many characters for discrimination that their identification 

 can be made with utmost certainty. 



The associated species, Graptodidya proava (Eichwald), has a similar 

 cribrose zoarium, but differs in its striated base pointed for articu- 

 lation. When this base is not present, the absence of mesopores and 

 the smaller, differently shaped zooecia of the Graptodidya are suffi- 

 cient to distinguish it. Stidoporella gracilis (Eichwald) is also a 

 cribrose species, but, as indicated under its description, the dimen- 

 sions of its zoarium and the shape of the zooecia are quite different. 



Fig. 53.— Stictopoeella ceibeosa. a and 6, tangentlvl A^^) 



VEETICAL SECTIONS, X24, OF A WELL DEVELOPED RUSSIAN 

 EXAMPLE IN WinCH MESOPORES .VEE LESS ABUNDANT THAN 

 USUAL. WaSSALEM BEDS (D3), UXNOEM, ESTHONIA. 



