353 
of the Ventriculidse of the Chalk. 
sions*. These divisions communicate internally, either by open- 
ing directly into each other, or by opening into a central cavity 
which they surround. 
This genus therefore presents theVentriculidse in a new light. 
With far less of the intricate complexity of fold of membrane 
which is found in the other genera, it exhibits what may, in con- 
tradistinction, be called a convolution of membrane varying greatly 
in different species. And that this convolution is a distinct 
thing from the fold already noticed will be evident, should its 
essential distinctness be not otherwise recognized, from the fact 
that several species have the fold as well as the convolution. In 
the descriptions which follow, the fold and the convolution will 
be distinguished as the ’primary and the brachial fold. 
This genus, like each of the others, will be found to have all 
its modifications adapted for the purpose of maintaining strength 
and stability of form and the free access of sea-water. We shall 
find some contrivances for these purposes of singular novelty and 
beauty ; and it is upon the marked distinction of two groups in 
one general arrangement for ensuring to the whole polypiferous 
surface a full and constant supply of the grand element of the 
existence of the creatures that the sectional division of the genus 
is founded ; — the one section having the separate lobes of such 
size, or so arranged with reference to a central cavity, that one 
maint entrance afforded sufficient access to the sea- water; those 
lobes in the other being so extended, or so arranged with reference 
to each other, that additional means were needful for that end. 
The roots do not, in general, differ in this genus from the same 
part in the others ; but I shall have occasion to call attention to 
some special and very remarkable contrivances in the arrange- 
ment of this part. 
The forms are all well-defined ; and though, like Ventriculites, 
specimens of such species as are not of great rarity are found of 
various sizes, there is little danger of confounding any two of the 
species unless in a very fragmentary state. As to the question 
of growth, the present genus may be considered, from the fact 
just mentioned, to stand on the same ground as the genus Ven- 
triculites. 
Different forms of this genus are found in the Upper, the 
Middle, and the Lower Chalk, and even in still lower beds of the 
* See pp. 46, 206 note J, and 293. 
t The very beautiful arrangement of B. angularis will be found to be one 
of those interesting apparent (at first sight) exceptions which prove a rule, 
the main access being through the central cavity, and the lateral perforations 
ensuring only the full and free circulation of the water entering, by that main 
access, the cavities of the ai-ms. The contrivance in all the group Aperti 
is of a very different character. See post, p. 361. 
Ann. ^ Mag. N. Hist. Ser. 2. Vol. i. 
24 
