56 
from the waters a silicious skeleton of surpassing beauty, as seen 
in the Eujylectella and Daet ulocalijx, or elaborates long bundles of 
silicious fibres, as in Hijalomma and Pliaronema. 
Another substance found in sponges, and that with which we 
are best acquainted, is keratode, or horny fibre, this like the 
silicious framework just alluded to, is invested with sarcode ; in 
some genera, (as in the sponges used for domestic puri^oses,) no 
spicides are formed, in others the spicules predominate. 
As we find among the Foraminifera certain forms which do not 
secrete a shell, but form one by glueing together minute grains of 
sand, so may we also detect certain species of sponge in which the 
skeleton has neither silica or solid keratose, but is composed of 
grains of sand enclosed in a thin keratose coverino" 
_ O 
I am much afraid that I have already occupied so much of your 
time with what is after all a very meagre outline of the structure 
of tliese remarkable organisms, that Avhat should be the principal 
subject of the paper must be treated very briefly. I must, how- 
ever, in as few words as possible call jmur attention to the proto- 
plasm found in layers on the ooze at the bottom of the sea ; those 
of us who attended the Biological Section of the British Associa- 
tion in 1868, will remember an interesting pajjer by Professor 
Huxlej- on Lathybius, in Avhich he described this sarcodous layer, 
and the remarkable forms occurring in it, known as coccoliths and 
coccospheres ; these forms may be found in chalk, clearly indicating 
that the bottom of the sea, during the cretaceous period, was also 
covered in ])laces Avith protoplasm. I must ask you to bear this 
fact in mind, as I think Ave shall find that it has an important 
bearing on the theory of the Spongeous Origin of Chalk Flints. 
If a thin chip or section of Flint is submitted to microscojiic 
examination, sponge sj)icules in more or less abundance, Avill inva- 
riably be seen, mixed Avith these Avill be found casts of the interior 
of the chambers of Foraminifera, fragments of Polyzoa, and small 
molluscous shells. 
When a recent sponge is examined, similar organisms Avill be 
seen entangled in the reticulated skeleton. A cretaceous Hint, like 
silica obtained by dialysis is non-crystalline, breaks Avitli a distinct 
conchoidal fracture, is singly refractive, and therefore is not all'ected 
by a polarized beam of light, in this resj)ect resend)ling silica 
taken u]) by uiuloulrtod animal or vegetable organisms. 1 may, 
