10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



as seen in young branches or in deep tangential sections of older 

 branches, have a different appearance than older stages of develop- 

 ment, as seen in shallow sections of older branches. In younger 

 stages, the dark, granular, zooecial-mesopore boundaries are easily 

 visible and in tangential sections appear as straight lines between 

 zooecia and straight or curved lines between zooecia and adjacent 

 mesopores. The curved boundaries are convex toward the mesopores, 

 producing irregular, compressed, subpolygonal cross sections in the 

 mesopores and subpolygonal to subcircular boundaries in the zooecia. 

 In these younger stages the mesopores are open, having little or no 

 laminated tissue on the mesopore walls inside the boundaries. The 

 mesopores appear to be merely open spaces between the zooecia. The 

 zooecia contain a thick laminated deposit inside the boundaries that 

 smooth out the polygonal configurations of the boundaries and pro- 

 duce generally oval zooecial voids. 



In tangential sections older growth stages are characterized by a 

 general thickening of skeletal material in zooecia and mesopores. 

 The dark zooecial-mesopore boundaries are largely obscured, zooecial 

 walls are thickened, reducing the diameters of zooecial voids, and 

 mesopores are generally filled with skeletal material originating 

 either from the mesopore walls or a combination of walls and dia- 

 phragms. The filling of mesopores makes their numbers obscure in 

 tangential sections in older growth stages. 



Acanthopores in tangential sections are randomly arranged on the 

 zooecial boundaries. Many appear transparent and structureless, a 

 few show the concentric laminae more typical of the Trepostomata. 

 Acanthopores are extremely variable in number and diameter from 

 zoarium to zoarium, and within a few zoaria are extremely variable 

 in diameter. Many acanthopores, regardless of diameter, cause in- 

 flection of zooecial walls. There seems to be little correlation between 

 zooecial wall thickness and acanthopore diameter, the tangential 

 sections through older growth stages with thick walls can have either 

 large, small, or variable acanthopores. This variability in diameter 

 of acanthopores is at least partly controlled by the notched character 

 seen in longitudinal sections. 



Monticules in tangential sections are either concentrations of 

 several filled mesopores or zooecia that are slightly larger in diameter 

 than intermonticular zooecia. 



Quantitative data. — The following tables are based on sections 

 from 25 fragmentary zoaria that are thought to be topotypes of 

 Nicholson's original specimens. All measurements are in millimeters. 



