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 
Diagram of a branching tre- 
postomatous bryozoan showing 
in black the colony as seen 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.) 
Fig. 
zoa- 
111. — Branching of 
rium. 
Longitudinal thin section of 
Tretocycloecia reticulata new 
species, X 12, showing that the 
separation between the two 
axial tubes of ramification oc- 
curred near the base of the 
cylindrical branches. 
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 1912. Lee, British Carboniferous Trepostomata, pi. 14, fig. 3, p. 144. 
