Mineral Deposit in the Rhizopods and Sponges. 81 
In the Acanthometrina which belong to another order, namely, 
the Protodermata, the plan of siliceous deposit is nevertheless 
essentially the same—the elongated spines (acanthostypes) never 
being tubular, as erroneously asserted by Professor Miller. The 
appearance of tubularity in these organisms, as in the Polycystina, 
is an optical illusion engendered by the longitudinal ribs of 
which the acanthostypes may be said to be made up*. 
In the Thalassicollide, which, together with the Dictyochide, 
are placed by me in the same order as the Acanthometrina, the 
plan of deposit is for the first time modified, but only to the extent 
of taking place in two directions from the axis of each spicule. 
In other words, the spicule is deposited within the sarcode en- 
tirely, but not in previously existing cavities or around stolons. 
Hence the silex is secreted only from within outwards. 
Lastly we arrive at the Dictyochide, a group I have found it 
necessary to set apart as a distinct family, owing to the fact 
of their presenting the solitary example of true tubular forma- 
tion amongst the whole of the Rhizopods. In the organization 
of their soft parts they are closely allied to the Acanthometrina 
and Thalassicollide ; whilst the tubularity of their siliceous 
framework, and its formation of two separate isometrical portions, 
at once stamp this family as the true connecting link between 
the Rhizopods and Sponges. It is a singular fact that up- 
wards of twenty varieties of these very common organisms have 
been described as distinct species on characters of no higher 
import than the number of spines or angles presented by the 
siliceous framework ; and that some have actually been described 
and figured as seen in a living condition with half of the internal 
skeleton deficient ! ' 
It only remains for me to add that in other genera of the 
Foraminifera than those referred to by Professor Schultze (as, 
for instance, in Globigerina), and. likewise in some of the Poly- 
eystina (Haliomma), specimens are not unfrequent in which the 
chambers are more or less choked up with entozootic sponge- 
growth ; whilst the chambers of Globigerina are at times filled 
with effete frustules of a free-floating pelagic surface Diatom, 
namely, Chetoceros. 
Hence, assuming the order of deposit of the mineral matter 
of all these structures to be constant amongst the members of 
the same family, the facts now advanced furnish the fullest 
* In the ‘Annals’ for October 1863, Mr. Carter describes, under the 
name of Acanthocystis turfacea, an organism recently found by him in De- 
vonshire, which he refers to the order Echinocystidia (Echinocystida ?) of 
MM. Claparéde and Lachmann. The spines in this form are said to be 
hollow; but, for reasons above given, this would at once remove it from 
Acanthometra, and indicate, in this respect, its close affinity to Acineta. 
Ann. & Mag. N. Hist. Ser 3. Vol, xii. 6 
