NORTH AMERICAN EARLY TERTIARY BRYOZOA. 637 



are parallel to each other and their length is unlimited. They give rise easily 

 to adventitious tubes. 



In transverse section all such tubes appear of the same size at the center 

 as at the periphery; the smaller tubes dispersed irregularly between the large ones 

 are the young tubes (fig. 197 B). 



In longitudinal section the interzooecial spaces are generally vesicular through- 

 out their course in the massive forms (fig. 197 A), but only at their extremity 

 in the ramose forms (fig. 197 C). However there are numerous exceptions to 

 this. These vesicles are sometimes quite large (Fig. 197 D) ; we are ignorant 

 of their function. There is no zone of growth visible, since the tubes have the 

 same diameter. 



The ancestrular tubes are short and not strictly parallel to each other. They 

 rise more or less in a spiral around the ancestrula and form the nuclear region, 

 with nonoriented tubes (=immature region of the Trepostomata) (fig. 197 A). 



Club-shaped tubes or typical Cyclostomata. These increase regularly in diame- 

 ter during the greater part of their length. They are slightly oblique to each 

 other and always have a part at the exterior of the zoarium. Their length is 

 limited. They rarely give rise to adventitious tubes. They become cylindrical 

 when near the zone of growth or when they bend toward the exterior, or finally 

 when they curve at a right angle with production of adventitious tubes. The 

 last case is without exception. 



In transverse section these tubes appear smaller at the center than at the cir- 

 cumference (fig. 198 A, B). In longitudinal section they are separated generally 

 by a linear space, rarely vesicular (fig. 198 C). 



The zone of growth is the ensemble of the small orifices of the incompletely 

 developed tubes; it is quite variable in form and size (fig. 198 D, E). The 

 lamella on which the new tubes are developed is called the basal lamella (=germinal 

 lamella of D'Orbigny). 



Tubes with facets (Mctopoporinn}. These are expanded tubes in which the 

 peristome is closed by a facet (D'Orbigny) or zooecial area (Levinsen). This 

 area (fig. 199) is perforated by an orbicular or semilunar orifice destined for 

 the passage of the tentacles. The orifice is often closed by a sort of calcareous 

 operculum. The zooecial walls are thickened at their extremity. 



This form of tubes has completely disappeared from the recent seas; they 

 correspond therefore to some anatomical features rather difficult to reestablish. 

 Such a form exists in many families: Melicerititidae, Lobosoeciidae, Plagioeciidae. 

 and other families. 



Tubes with rhomboidal orifice. These are cylindrical or funnel-shaped. Their 

 aperture is oblique to the zooecial axis; it is not orbicular, but is elongated and 

 rhomboidal (fig. 200). The bundles are caused by the reunion of many tubes and 

 form zoarial and lateral expansions symmetrically arranged, called pinnules. They 

 comprise many genera of the family Cytisidae. This form of tubes has disappeared 

 from the recent seas. 



