9 
The accommodation for circular corallites in this condition is simple, 
and consists of a more or less developed irregularity of septal structure 
in the angles of would-be polygonal corallites. Rarely a few extra columns 
are seen in such areas in section; these have all the appearance of septal 
ridges but do not lie on the periphery of the corallite. A single specimen 
collected by Lauge Koch from cape Calhoun (M.M.K. 431) shows this 
structure multiplied considerably. It is discussed below in the considera- 
tion of Troedsson’s material and is referred to this species, not to C, 
Nicholson noticed some intramural va- 
cuities in his material (1879, 160). Most of 
the sections of C. canadensis examined by 
the writer show these vacuities (Plate II, 
figure 7). They do not always occur in the 
wall, but are usually found near it. In 
transverse section they are circular or oval 
and in longitudinal section appear as shapes 
varying from circular to inflated vermiform. 
Usually they are occupied by transparent 
calcite and show no structure; their bound- 
aries are dark; occasionally, however, they 
are filled in with opaque (muddy) material. 
These vacuities may open as small tubes on 
the surface of the corallum. Nicholson’s 
comparison with similar structures seen in 
Pleiirodictyum cannot be carried far, for in that case the tubes were formed 
prior to the coral skeleton, which was modified to conform with their pres- 
ence (1879, 147). But here the vacuities may traverse septal structures 
which were obviously there before them. As a rule, however, the vacuities 
occur more commonly in the porous parts of the corallite “wall.” Their 
irregular occurrence shows definitely that they are not part of the coral 
structure, a fact borne out by a complete absence of them in the Akpatok 
Island and Cape Calhoun examples of Calapoecia, They suggest, more 
than anything that has come to the waiter’s notice, the presence of some 
form behaving like the Recent polychaete Leucodora, which perforates the 
corallum of Heliopora (Hickson 1924, 119-120). This modern form, how- 
ever, secretes a calcareous tube, which structure is apparently absent (or 
at any rate unpreserved) in the perforations in these Calapoecias. If the 
vacuities have such an origin, the invading organisms must have been able 
to destroy calcium carbonate in some way, for as already mentioned the 
septal structures may be broken. i The vacuities do not extend parallel to 
the corallites very far. The occurrence of such vacuities in Lyopora 
has not been observed in any of the specimens examined by the writer. 
Nicholson designates them “ intramural canals ” in his description of Plate 
Vn, figure 2b, but since they are so apparently not part of the original 
structure of Calapoecia canadensis it would be better if they were known 
by some name conveying less association with the structure, such as “ dis- 
ruptive canals.” 
^ Professor Keiliu, who kindly examined sections showing these vacuities, said he 
knew of no modern para-site that was capable of behaving in such a manner and suggested 
that perhaps bacteria might be responsible for the vacuities. 
borealis as he suggested. 
Figure 1. Calapoecia canadensis 
Billings. Enlarged drawing of 
part of a weathered specimen 
from Manitoulin island, lake 
Huron, Ontario. B-M. R21o37. 
