NO. 6 BRYOZOAN GENUS TREMATOPORA — BOARDMAN O, 



located single pores that penetrate the diaphragms at right angles. 

 In longitudinal thin sections that pass through these pores, diaphragms 

 display transversely curved laminae that continue uninterrupted to 

 the pores. The curved laminae immediately adjacent to the pores 

 define the rounded boundaries of the pores. 



If walls of adjacent mesopore chambers are longitudinally lami- 

 nated, generally the wall of the earlier chamber is connected directly 

 with the curved laminae on the proximal side of the intervening dia- 

 phragm and the wall of the later chamber is connected with the 

 distal side of the diaphragm. If walls of adjacent mesopore chambers 

 are formed by transversely curved laminae, the diaphragm and ad- 

 jacent walls will appear to be a continuous unit, or the diaphragm is a 

 direct continuation of the proximal wall and the wall of the distal 

 chamber is discordantly joined to the distal side of the diaphragm. 

 Rare, isolated mesopore diaphragms display complete continuity with 

 the walls of distal chambers. 



In longitudinal thin sections, mesopore diaphragms in which the 

 pores were not intersected appear longitudinally laminated. Com- 

 monly the diaphragms are compound ; the proximal half of a dia- 

 phragm is continuous with the wall of the preceding mesopore 

 chamber, the distal half is continuous with the wall of the succeeding 

 chamber. Other variations in diaphragm-wall relationships are less 

 common ; the two parts of the compound diaphragm can be unequal 

 in thickness, or in extreme development a diaphragm loses its com- 

 pound appearance and is wholly continuous with the preceding or very 

 rarely the succeeding chamber walls throughout or at either end. 



Outer region of mesopores. — In the outer region of the exozone, 

 mesopores are not beaded and the walls and diaphragms display ex- 

 treme thickening. This greatly thickened skeletal growth can begin 

 on the distal side of the last thin diaphragm, the laminae covering the 

 central pore of the thinner diaphragm and curving distally into the 

 mesopore walls, or it can begin by an abrupt thickening of the meso- 

 pore walls. Diaphragms in this outer region are extremely variable 

 in thickness and spacing. A single diaphragm, greater in thickness 

 than the diameter of the enclosing mesopore, can correspond in thick- 

 ness and position with a series of irregularly and closely spaced dia- 

 phragms in adjacent mesopores. Most diaphragms are planar, but a 

 few are strongly curved and join adjacent diaphragms before reaching 

 the mesopore wall. The last diaphragms that were formed are in the 

 distal ends of the mesopores so that in external view the walls and 

 diaphragms of the mesopores combine to form the very shallow polyg- 

 onal depressions between the zooecia. 



