NORTH AMERICAN EARLY TERTIARY BRYOZOA. 

 GEMMATION. 



647 



In principle, gemmation is independent of the form of the tubes and their re- 

 union in bundles. Gemmation may be divided into juxtaposed, peripheral, or 

 oriented. 



In juxtaposed gemmation the tubes are parallel and open at their two ex- 

 tremities. In transverse section, they are larger at the center of the zoarium. 

 Such gemmation exists only in the family Corymboporidae, which became extinct 

 in the Cretaceous (fig. 209 A). 



FIG. 209. Methods of gemination. 



Sketches showing (A) juxtaposed, (B) peripheral, (C) triparietal, 

 (D) intrazoarial, (B) dorsal, and (F) axial methods of gemination. 



Peripheral gemmation results from the bifurcation of the tubes at all heights 

 and in all directions. It is observed most frequently in bryozoa with cylindrical 

 tubes, (Trepostomata, Ceriopora), but it has been noted in genera with other forms 

 of tubes (Ceriocava). 



Oriented gemmation occurs in a definite manner on a single or on two sides 

 of a basal lamella or of an axial zone. It is triparietal or biparietal. 



In triparietal gemmation each tube in longitudinal section is, at its base, 

 adjacent to three other tubes. The latter are always short. This kind of gem- 



