74 W. D. Lang—Caleiwm Carbonate 
adjacent zocecia are completely confluent. Cumings and Galloway 
show that this distinction does not hold—that Lee had already 
remarked that the presence or absence of the dark line was inconstant 
—and their researches correlate the presence of the dark line with 
the thinness of the growing edge of the wall and the consequent 
‘‘ steep pitch” of the growth-lamine. ‘‘ For some reason, wherever 
the wall lamine of the trepostomes are sharply bent the material 
appears dark ... It is probable that the size and arrangement of 
the minute granules of which the wall laminz are composed differ 
slightly at such points from the normal size and arrangement in 
other parts of the wall. In fact, in well-preserved material and in 
very thin sections it can be shown that this is actually the case.” 
The similarity of the Trepostome wall with that of the Brachiopoda 
when they are viewed in thin sections is pointed out, and ‘‘in such 
sections as are shown in figures 45 and 49 it amounts almost to 
identity’. 
Perhaps the most interesting part of the paper concerns the 
Cingulum or secondary thickening of the wall, and its relation to 
the secretion of diaphragms and cystiphragms which are clearly 
shown in the figures to be continuations of this secondary thickening. 
The authors regard the thickening as an old-age character. ‘‘ The 
thickening of the interzocecial walls, due to the development of the 
cingulum, is often very great—far greater than would merely com- 
pensate for the increasing separation of the zocecia, as they extend 
radially outward from the axial region. There is an actual reduction 
\in size of the zocecial chamber; indeed, in some cases, an extreme 
reduction (figures 1 and 17). We believe that this extreme develop- 
ment of secondary deposits is a senile feature, analogous to the great 
thickening of brachiopod shells and the shells of the Mollusca in old 
age. In recent Bryozoa the zocecia of the older portions of zoaria 
often become almost or quite filled up with stony deposits, and it seems 
that the ectosare [ectocyst] may continue to secrete such deposits 
after the polypide has wholly disappeared from the zocecium.”’ 
Let us go further. Say that some metabolic process, such as one 
involved in nitrogenous excretion, resulted in the precipitation of 
calcium carbonate in the tissues or upon the surface of a marine 
organism—Mollusc, Brachiopod, or Polyzoan, and was turned to 
useful account as affording a supporting or protecting skeleton or 
shell; that this production of calcium carbonate became increasingly 
constitutional so that the mere need for a skeleton or shell was more 
than met; that the process could not be arrested or countered in 
all the organisms that had acquired it, and in these the disposal of 
superfluous calcium carbonate became a pressing problem; that, 
finally, those organisms that found no way out of the difficulty were 
doomed to extinction under a mass of calcium carbonate of their 
own making. 
At some time or other most animal phyla have met the calcium | 
carbonate problem in an acute form. The Protozoa can always crawl 
away from their skeleton, and many organisms that build tubular 
shells can similarly move up their tubes which thus become of 
indefinite length, while tabule, diaphragms, etc., are merely the 
