356 Mr. Toulmin Smith on the Classification 
fold : brachial fold developed very rapidly from the base, in 
broad and deeply undulating convolutions, so deep as to leave 
no regular central cavity; most prominent surfaces much 
flattened, and spread laterally till those of adjoining convolu- 
tions meet and unite in many places, often presenting, towards 
the lower part of the whole, a continuous surface. 
This species displays many peculiarities. The structure of its 
simple membrane seems coarse, that is, the squares are larger 
than in any other species of the Ventriculidie. The root is ex- 
ceedingly short. The most curious point, however, is the union 
of the flattened prominences of the convolutions. A reference 
to fig. M, p. 354, will show, in B. tuber osus, a disposition to 
flattening of the most prominent part of the convolution in 
that species, and will make the nature of the present fold easily 
understood, where that flattening is so much more extensive 
that adjoining convolutions meet and unite*. This peculiarity 
was important in the present species on account of the great 
depth of the convolutions, which would, without it, have been 
more liable than in B. eleyans and other species to be displaced, 
and so injury to have happened to the polyps. But a curious 
phsenomenon is often presented in consequence. The flint, — 
which, from its specific gravity, w^ould always lie, when fluid, 
near the sea-bottom, — was attracted round the base of the spe- 
cimen, but, the surface there being usually continuous for a con- 
siderable distance, access to the lower part of the inside seldom 
took place, unless the siliceous fluid was very abundant ; any silex 
attracted towards the inside f in the manner before suggested 
(p. 85) being specially attracted by the convolutions interposing 
before it reached the base, solidifying there, and thus preventing 
the flow of other fluid towards the base. Hence, on the decay 
of the animal matter, a large hollow was frequently left in the 
flint, which is often now found only partially filled up by chal- 
cedony. Such flints are cup- shaped, very regularly rounded be- 
low, and have a flat top. Near the edge of that top a continuous 
line is usually found, showing the place of the united mem- 
brane. Within this are seen traces of the convolutions of the 
* This outer surface, where the polyp-skin is preserved, which very rarely 
happens, may sometimes appear as if apolypous. A very careful comparison 
of ail the specimens I have on which remains of this membrane are found, 
leads me, however, to the conclusion that this is not the fact ; but that, the 
membrane being coarser than usual, the pt)lyp-cells, though present, are not 
so clearly preserved as in some other species. 
t The access of the liquid chalk was often made equally difficult by the 
collapsion, after death, of some of the lower convolutions on one another. 
Hence we find the bases of specimens enveloped in that substance often hol- 
low like those in flint. Some deposit of chalcedony has often taken place 
in those hollows, as in the hollows of the flints. 
