EAELY PALEOZOIC BEYOZOA OF THE BALTIC PROVINCES. 293 



region where the walls are thin, and semidiaphragms are far apart, 

 and a mature zone conspicuous for its crowded partitions and slightly 

 thicker walls. Two such layers are shown in the accompanying 

 illustration (fig. 178 d), while a more magnified view of a mature zone 

 and part of an immature zone is shown in figure 178 c. The occur- 

 rence of incomplete diaphragms throughout the zooecial tubes and the 

 practical absence of small, beaded tubes with straight partitions 

 may be seen in such sections. The arrangement and abundance 

 of the semidiaphragms is about the same in this species as in the 

 other massive forms, H. rotundatum and H. pygmseum, but an especial 



Fig. 179.— Hemiphkagma subspheeicum. Four views of zoael*., natueal size, a, side view of a 



SPECIMEN COMPOSED OF TWO EOUNDED ZOAEIA GEOWING TOGETHEE; 6, SIDE VIEW OF A LONGEE, MOEE 

 FLATTENED SPECIMEN; C, A EOUNDED, SOMEWHAT lEEEGULAE EXAMPLE; d, BASAL VIEW OF SPECIMEN, 

 SHOWING CICATEIX OF GASTROPOD UPON WHICH GROWTH COMMENCED. WESENBEEG LIMESTONE (E), 



Weseitberg, Esthonia. 



point of difference is the crenulation of the tubes in H. subspJiericum. 

 This latter feature is likewise shown in vertical fractures of the 

 zoarium. 



Tangential sections bring out the extreme simplicity and thinness 

 of the zooecial walls and the absence of acanthopores. Mesopore- 

 like spaces are present in such sections, but these are usually young 

 zooecia. At the surface of the zoarium such spaces are practically 

 wanting, the zooecia being regularly polygonal and in contact on all 

 sides. The semidiaphragms, which in vertical sections are noted 

 to frequently curve slightly downward, are seen in tangential sections 

 where they appear as a more or less curved line crossing the zooecium. 



