Mr. Toulmin Smith on the Ventriculidse of the Chalk. 203 
XXIII. — On the Ventriculidse of the Chalk; their classification. 
By J. Toulmin Smith, Esq. 
[Continued from p. 48.] 
Descriptions of species. 
Family Ventriculid^. 
Character. Structure. Polypidom membranous : membrane com- 
posed internally of fibres arranged in several — usually five* — 
layers of cubic squares, equal, for the most part, in the plane of 
thickness and of superficies, and connected at all their angles 
by other fibres having a regular octahedral arrangement : exte- 
rior to this, both within and without, a dermis composed of a 
single layer of smaller squares, and in which the polyps are 
lodged, usually on both surfaces : exterior to this a simple epi- 
dermis : roots distinct, less regular in structure and without oc- 
tahedral fibre. 
Habit. One or more central cavity, the principal opening to 
which is at the top : roots ensheathing base of polypidom and 
extending below into radicles ; never affixed to solid bodies. 
The details which have been already given render further ob- 
servations on the characters of the family unnecessary. 
Genus Ventriculites. 
Character. Pouch-shaped; varying greatly in size and dilata- 
tion : cavity single and regular : membrane forming the wall 
of the cavity either simple and smooth on both surfaces or 
more or less closely and regularly folded (thus giving it a ru- 
gose character) : margin of wall thinned or rounded ofi* to an 
edge : polypiferous on both external and internal surface. 
The structure of all the Ventriculidse is obviously designed for 
the purpose of securing permanence of form, and thus safety and 
free access of water to all the individual polyps. This object is 
effected by two means : first by the very remarkable structure of 
the membrane already described ; secondly, by the regular, often 
nearly hemispherical figure which the whole body assumes. 
Specimens of Ventriculites are found of all sizes, and it can 
easily be understood that, from the earliest period of their deve- 
lopment, the same, general form is assumed, which, as they increase 
and spread, is still retained. Thus the question of their growth 
* I believe it to be always five ; but the difficulty which exists, from the 
causes already named, in ascertaining these minute points in all individual 
instances induces me thus to qualify the generality of this character. 
