330 



BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



In tangential sections the zooecia are polygonal and in contact 

 with each other, usually on all sides. The walls are comparatively 

 thick, and show the original boundary line between the zooecia as a 

 well defined dark line. Mesopores are few and limited almost entirely 

 to the maculae. 



Vertical sections are particularly interesting in showing the dis- 

 tribution of diaphragms and mesopores. As shown in figure 206 e, 

 the latter originate in the bend from the immature to the mature 

 zones, but frequently cease before the surface is reached. In some 



Fig. 206.— Hallopoea splendens. a, fragmentary specimen, natural size; 6, tangential section, 



X20, CUTTING THE MOST MATURE PORTION OF THE ZOAEIUM; C, USUAL ASPECT OF TANGENTIAL SECTIONS, 

 X20; d, VERTICAL SECTION, X20, WITH PRACTICALLY NO MESOPORES; 6, ANOTHER VERTICAL SECTION, X20, 

 ILLUSTRATING OCCURRENCE OF MESOPORES. JEWE UMESTONE (D1), BaRON TOLL'S ESTATE, ESTHONIX. 



cases (see fig. 206 d) mesopores are practically absent, even in this 

 restricted zone. As usual the diaphragms are closely arranged in the 

 mesopores. In the zooecia proper diaphragms are equally abundant 

 as in the mesopores, in the earliest stages when the tubes are scarcely 

 wider than the mesopores. After the zooecia have attained their 

 normal size, diaphragms are from one to three diameters distant 

 from each other in the immature zone, but crowded in the mature 

 region where 8 to 10 mm. may sometimes be counted in the length 

 of one diameter. 



