14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



boundaries are well-defined dark lines or narrow serrated zones and 

 the wall laminae that intersect the boundaries form either a 

 V-shaped or sharply U-shaped pattern. As the walls are followed 

 distally in the early exozone, the V-shaped pattern is generally lost 

 and the U-shaped pattern broadens so that the zone of curved laminae 

 makes up an increasing proportion of total wall thickness. Also, as 

 seen in longitudinal sections, zooecial boundaries are commonly 

 marked by a dark granular zone of appreciable width that can al- 

 ternate at irregular intervals along a zooecial wall with the more 

 common boundary formed by abutting laminae. As the broadening of 

 the U-shaped laminae progresses distally, the zooecial boundaries be- 

 come more obscure, until the boundaries are completely lost in later 

 growth stages. 



There is a general increase in zooecial wall thickness distally in 

 the early exozone as the zone of curved laminae broadens. Zooecial 

 wall thickness is markedly variable, however, from zoarium to zoarium 

 and also can vary irregularly along the length of a zooecium in the 

 outer part of the exozone. The thickness of a limited segment of a 

 zooecial wall in the outer exozone can decrease to thicknesses common 

 in the early exozone, but the zone of curved laminae remains relatively 

 broad and the zooecial boundary obscure, indicating that wall thick- 

 ness alone does not determine the nature of the curved laminae and 

 the zooecial boundary. The pattern of laminae is V-shaped and the 

 zooecial boundaries well defined throughout the length of a few 

 zooecia in two of the specimens sectioned. 



The general aspect of tangential sections varies considerably with 

 zooecial wall thickness from zoarium to zoarium and also seems to be 

 controlled, at least in part, by stage of ontogenetic development 

 within a zoarium. In the five sections containing exozones of 3 mm. 

 or less in width, the corresponding tangential sections display rela- 

 tively small acanthopores that cause little or no inflection of zooecial 

 walls, and the zooecial walls range from integrate to broadly amalga- 

 mate. In the 10 sections containing exozones ranging from 3 to 5 mm. 

 in width, acanthopores range in size from small, to medium — 

 recognized by noticeable inflection of zooecial walls, to large — recog- 

 nized by acanthopores controlling cross-sectional shapes of mesopores 

 and zooecia by strongly inflecting their walls. Zooecial walls range 

 from narrowly amalgamate to broadly amalgamate in the 3- to 5-mm. 

 interval. Also within this interval, the three sections showing medium- 

 sized acanthopores are all amalgamate, and the one section that has 

 large acanthopores is broadly amalgamate. In the five sections having 

 exozone widths greater than 5 mm., corresponding tangential sections 



