EAELY PALEOZOIC BRYOZOA OF THE BALTIC PROVINCES. 283 



The most important internal features are shown in vertical sections 

 where the thin, irregularly fluctuating walls and the rather numerous 

 incomplete diaphragms, with as many as three in a tube diameter in 

 the mature zone, are the most important points to be noted. The 

 zocecial walls in the mature zone are but slightly thickened, differing 

 in this respect from nearly all other ramose species of HemipTiragma. 

 The particular thin section figured shows incomplete diaphragms at 

 distances ranging from one to two tube diameters in a portion of the 

 immature region. 



In general zoarial and zocecial characters these Russian examples 

 are practically identical with the abundant American form from the 

 Chtambonites and Nematopora beds of the Lower Trenton in Minne- 

 sota. Indeed, the only difference noted at aU was the occurrence of 





Fig. 171.— Hemipheagma tentjimtjeale. Ulkich's views of the Minnesota types, o, sueface of 

 specimen showing the semidiaphragms in the zocecial cavities, x9; 6, vertical section, x18; 

 c, tangential section, x18, showing the fully matured condition. lowee beds of the 

 Trenton limestone, near Cannon Falls, Minnesota. 



a few incomplete diaphragms in the immature zone of the Baltic 

 specimens. Such an occurrence is known in old examples, particu- 

 larly at the base of a zoarium, and it is evident that the particular 

 sections figured were obtained from such specimens. 



The ramose habit of growth, thin-walled, polygonal zooecia, few 

 mesopores, and especially the numerous incomplete diaphragms of 

 the mature region, are characters which wiU distinguish H. tenui- 

 murale from associated forms. Comparison of figures 170 and 171 

 will show the practical identity of internal structure in the Russian 

 and American specimens. 



Occurrence.- — Common in the Clitambonites and Nematopora beds 

 of the Lower Trenton at various localities in Minnesota and Iowa. 

 Rare in the Wassalem bed (D3) at Uxnorm, near Reval, Esthonia. 



Plesiotype.—C&t. No. 57416, U.S.N.M. 



British Museum, a thin section of the Russian type. 

 92602°— Bull. 77—11 ^20 



