EAELY PALEOZOIC BRYOZOA OF THE BALTIC PROVINCES. 295 



from either side of the tube half way across the cavity. Occasionally 

 a complete diaphragm seems to be present, but this is merely an 

 incomplete one cut parallel to its edge instead of across it. These 

 partitions are placed at a rather uniform distance of half a tube diam- 

 eter apart in the more crowded or mature zone. The second notice- 

 able feature of vertical sections is the occurrence of well-marked 

 mesopores crossed by normal, entire partitions at intervals of their 

 own diameter. These mesopores are regularly beaded tubes as long 

 as their diameter is less than that of the ordinary zooecia, but as soon 

 as the size of the latter is reached, their walls become straight and 

 semidiaphragms are introduced, showing that the mesopores have 

 been replaced by zooecia. 



In vertical sections the present species is quite similar to Hemi- 

 pliragma pygmsBum from the Chasmops limestone of the island of 

 Oeland. Tangential sections show that the latter has considerably 

 smaller zooecia, thicker walls, and much stronger acanthopores. 

 Again, the zoarium of H. pygmmum, although subglobular in shape, 

 is of small, pedunculate masses. Another massive species in the 

 Baltic Ordovician is the H. suhspTiericum from the Wesenberg forma- 

 tion, but its large, thin- walled, angular zooecia, with few mesopores 

 are quite distinct from those of both the above forms, although again 

 the vertical section is somewhat similar. 



Occurrence. — Rare in the Orthoceras limestone (B3), island of 

 Rogo, near Baltischport, Esthonia. The species also occurrs in the 

 Echinospherites limestone (Cl) at Reval. Specimens are also known 

 from the Kuckers shale (C2) at Reval, and from Baron Toll's estate, 

 Esthonia. 



Holotype.—C3it. No. 57424, U.S.N.M. 



Thin sections of a specimen from the Orthoceras limestone, island 

 of Rogo, are in the collections of the British Museum. 



HEMIPHRAGMA MACULATUM, new species. 



Text fig. 181. 



Zoarium of small, slender, cylindrical branches 2.5 mm. in diam- 

 eter on an average. Surface without elevations but conspicuously 

 marked by large, distinct maculae 2.5 mm. apart, measuring from 

 center to center. These maculae, which are composed entirely of 

 small inesopores, are frequently several milKmeters in diameter, and 

 as they are shghtly depressed and are arranged alternately the 

 branches are irregularly constricted along a spirally ascending Hne. 

 Zooecia polygonal, six in 2 mm., with rather thick walls. Mesopores 

 angular, numerous but exceptionally small. Acanthopores absent. 



Figure 181 6 and c shows the usual view in tangential sections. 

 The unusually small mesopores, the thickness of the walls in both 

 kinds of cells, and the absence of acanthopores, are most character- 



