177 
The spines of the tubarium are numerous, fairly flexible, 
slender, straight, occasionally forked. They measure °3 to ‘4 
mm. in thickness, and they taper but slightly ; they frequently 
attain a length of 13 mm., although the majority are shorter, 
and measure 9 or 10 mm. beyond the surface of the tubarium. 
The spines are darker in tint than the general test, and can be 
traced into the latter for a considerable distance. The spines 
are solid; the superficial layer has a deeper tint than the 
central part, so that on a casual examination the spines appear 
hollow. Critical inspection, however, shows that the spine- 
axis has been developed intermittently by successive addi- 
tions to its extremity, the increments being all of the same 
shape, and approximately of the same size. The successive 
apices are, with the exception of the first few at the embedded 
base of the spine, all of the same width, and this accounts for 
the width of the spine remaining the same all the way along 
(see figs. 9-11, plate 3). 
The ostia or apertures of the tubarium are numerous, fairly 
closely set and uniformly scattered over the lateral and ter- 
minal surfaces of all the branches except the basal ones (fig. I, 
plate 2). They are oval in shape and measure I-2 or I°3 mm. 
by Imm. across. In fig. 7, plate 3, is shown an end view of a 
branch, the view that one would obtain when looking in the 
direction of the axis of the branch. The pear-shaped areas 
are the ostia, the small circles are the stumps of spines, cut 
short in order that their relations to the ostia may be studied. 
At a and 0 it will be seen that there is one spine between two 
ostia, but in the other cases each tube has a spine of its own. 
Except in the case of terminal groups, such as that shown 
in fig. 7, plate 3, the spines are not at first sight definitely 
related to the ostia ; as a rule each ostium has one, perhaps 
two, or even three spines arising from its margin, but some of 
the spines project form the surface of the tubarium between 
the ostia. The spines that arise from the margin of the ostium 
may arise from any part of that margin, 7.e., not necessarily 
towards the axis of the branch, nor on the peripheral side, 
nor laterally as regards the ostium. Sometimes two tubes 
project together from the general surface of the tubarium as 
a double peristome, and these may have a single spine between 
them, or may have two, or three spines. The number of 
spines on a branch is probably equal to the number of ostia, 
but one can only in certain cases recognise a definite relation 
between tubes and spines. 
The tubes occupied by the polypides are all blind at their 
deep ends, and each is occupied by one polypide and its buds, 
so that the species Cephalodiscus gilchristt comes within the 

