NORTH AMERICAN EARLY TERTIARY BRYOZOA. 
647 
GEMMATION. 
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). 
D 
Fig. 209. — Methods of gemmation. 
Sketches showing (A) juxtaposed, (B) peripheral, (C) triparietal, 
(D) intrazoarial, (E) dorsal, and (F) axial methods of gemmation. 
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, C eriopora ) , 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 land of gem- 
