NORTH AMERICAN EARLY TERTIARY BRYOZOA. 



649 



had induced Lee 1 to formulate his theory of reabsorption which is summarized 

 in figure 210. 



Branching. Branching of a zoarium is not a spontaneous accidental or rapid 

 phenomenon in close relationship with the mechanical movements of the water, 

 but is a true physiological phenomenon. Its object is the increase of the zoarial 

 surface, that is in reality, the surface of oxygenation. Moreover, as seen in 



H 



FIG. 211. Branching of zoa- 



riuin. 



Longitudinal thin section of 

 Trctocycloecia rcticuhita new 

 species, X 12, showing that the 

 separation between the two 

 axial tubes of ramification oc- 

 curred near the base of the 

 cylindrical branches. 



FIG. 210. Growth of zoarinm. 



Diagram of a branching tre- 

 postomatous bryozoan showing 

 in black the colony as seeu in 

 longitudinal section, in stip- 

 pling the terminal portion of 

 the same during an earlier 

 period and supposed to have 

 been resorbed, and in broken 

 lines, the terminal portion as it 

 might have been had growth 

 continued. (After Lee, 1912.) 



sections (fig. 211), the divergence of the axial tubes of ramification begins early, 

 almost at the base of the primitive branch. 



The necessity of the dichotomous, arborescent, or bushy architecture requires 

 the formation of accessory tubes of padding and of consolidation ; they often oppose 

 the free development of the poorly placed zooecia, the degenerate polypide of 

 which and the orifice are closed by lamellae. 



During the Paleozoic era the atmospheric pressure may have been greater 

 and consequently the quantity of oxygen dissolved in the sea water was greater. 

 In the following eras the rarification of the oxygen has caused an increase in the 



1 1012. Lee, British Carboniferous Trepostomata, pi. 14, fig. 3, p. 14-1. 



