357 
of the Ventriculidse of the Chalk. 
FiV. N. 
brachial fold which attracted and retained the siliceous fluid in 
its descent towards the base. Fig. N is 
a small specimen of this kind, instances 
which are not uncommon in some places. 
Fig. 5. PI. XV. represents a specimen 
which has been longitudinally divided, 
and has afterwards had the convolutions 
cleared out by the needle. The white 
central parts, round the entire edges of 
which the structure is seen, are the only 
parts where the true central cavity is cut, 
notwithstanding the section. The depth 
of the convolutions will be clearly seen. 
It will also be observed that there are several places where the 
flattened convolutions do not unite, thus leaving ample means 
for the free admission of sea-water to bathe the whole inner 
portion of what is really the external surface of the membrane, 
though so much surrounded by the overspreading flattened, and 
thus actually outer, surface. The true inner surface is bathed 
by means of the access of water through the upper part in the 
usual way. 
Specimens vary greatly in size. I have them from an inch to 
at least eight inches in height. The height to which the flat- 
tened outer surface is continuous varies in difierent specimens, 
being, as might be anticipated, greater in large ones. It is some- 
times much greater on one side than on the other, a circumstance, 
however, which, in the living creature, would not at all interfere 
with the free access of the sea-water, inasmuch as the communi- 
cation was free all around within and under this expanded con- 
tinuous surface. 
The species is found both in Upper and Middle Chalk. 
4. Brachiolites angularis. 
IMembrane exceedingly flne in texture, having a primary fold of 
minute and corresponding depressions arranged in more or 
less exact quincuncial figure : brachial fold expanding very ra- 
pidly into a varying number of arms opening into a central 
open cavity ; each arm having the two walls parallel and flat ; 
closed at upper and lateral edges ; terminating at the external 
angle in a broad triangular lip depressed in the middle ; and 
having at regular intervals, along its lateral edge, complete 
perforations, between which distinct and single root-fibres are 
attached to the membrane at intervals from the base upwards. 
This is certainly the most extraordinary and interesting of the 
whole family of Ventriculidse. It presents some points of such 
