NO. 5 ORDOVICIAN BRYOZOA — BOARDMAN 9 



to originate at transverse partitions, either one large zooecium becom- 

 ing two at the partition, or two zooecia becoming one. 



In the exozone, the species is characterized by an irregularity of 

 gross structures and poorly defined microstructure that is uncommon 

 in trepostomes. Walls between adjacent zooecia or adjacent zooecia 

 and mesopores are delimited by boundaries that in longitudinal sec- 

 tions are well defined, dark, granular, slightly serrated lines or zones. 

 The boundaries are located at the points at which laminae from 

 adjacent walls abut. Laminae of zooecia and mesopores are generally 

 difficult to distinguish and intersect the boundaries at angles of less 

 than 90 degrees to form the V-shaped pattern characteristic of the 

 genus. Diaphragms in the zooecia are generally thin, laminated, 

 slightly curved to planar, and widely and irregularly spaced. Cystoidal 

 diaphragms are few. 



Mesopores are common between zooecia although they rarely sur- 

 round the zooecia completely. The mesopores contain thick, closely 

 spaced diaphragms that are generally curved convexly outward. 

 Larger mesopores have more than one longitudinal row of dia- 

 phragms ; the rows are separated by the diaphragms themselves curv- 

 ing back proximally through 90 degrees to form an irregular, flattened, 

 cystose pattern. The proximal sides of mesopore diaphragms dis- 

 play a dark granular layer of material similar in appearance to the 

 material of the zooecial walls in the endozones and the zooecial 

 boundaries in the exozones. This dark layer is connected directly 

 to the boundaries between the mesopores and adjacent zooecia in 

 many mesopores. The distal sides of the mesopore diaphragms are 

 thicker than the proximal dark layer and are composed of laminated 

 material similar in appearance to that of the zooecial walls. The 

 laminated material thins rapidly as it bends either proximally with 

 the diaphragms in the center of a mesopore, or distally along the wall 

 of a mesopore. Thus, the laminated material on the mesopore side 

 of the wall is either lacking or much thinner than the laminated de- 

 posit of relatively constant thickness on the zooecial side of the 

 boundary. 



Acanthopores are conspicuous in longitudinal sections and char- 

 acteristically display a large core of structureless, transparent calcite. 

 The cores have sharply notched sides caused by the unequal exten- 

 sions of laminae into them from the surrounding laminated material 

 of the zooecial walls. A few acanthopores appear to have very short 

 lateral branches. 



In tangential sections, younger ontogenetic stages of development. 



