Structure in the Devonian Limestone, ce. 417 
the “cap of liberty,” compressed, conical, a little bent upon 
itself and formerly hollow, as indicated by a round plug of 
chalk, 14 inch in diameter, in the centre of the large or free 
end, the smaller one having been the point of attachment; 
surface regularly cancellated, with interstices 1-24th inch in 
diameter), and find that by subjecting a portion to the dis- 
solving influence of dilute nitric acid, a lithistid structure is 
brought out, in which part is chalcedonic and part calcareous, 
the latter in many instances having disappeared, while the 
former as often remains as a fragment of the original trifid 
lithistid spicule—just as in Prof. Zittel’s case (‘ Annals,’ 
1877, vol. xx. p. 264). Here, then, it is perfectly evident that 
the originally siliceous spicule of the lithistid has, to a certain 
extent, been replaced by calcareous material. 
At the conclusion of my description of Mr. Thomson’s fossil 
sponges from the Carboniferous Limestone of §8.W. Scotland 
(‘ Annals,’ 1878, vol. i. p. 141) it is stated that, not only the 
chalcedonized spicules of Hyalonema Smithit present rhombo- 
hedral excavations, but the casts of turreted shells which 
are composed of chalcedony present the same phenomenon ; 
and it might be added that, on the fretted-out surface of a 
large block of Stromatopora from the Devonian Limestone, I 
have just found casts of turreted and bivalve shells composed 
of calcite, which also present this rhombohedral excavation. 
Here, then, we have three states, viz. :—-1, the original sili- 
ceous sponge-spicule chalcedonized ; 2, the casts of shells in 
chalcedony ; and 3, the casts of shells in calcite, all presenting 
the same kind of rhombohedral excavations; and striking as 
it appears that the chalcedony and the calcite should respec- 
tively present the same kind of rhombohedral excavation, weare 
impelled to the conclusion, on the one hand, that the chalcedony 
is encroached upon by the calcite; while, on the other hand, 
the calcite, as a matter of course, weathers out in the form of 
its natural crystallization. I omitted to mention, in connexion 
with the former, that the fossilized spicules of Hyalonema 
Smithii do not present any of this double composition in 
the undecomposed Carboniferous Limestone, where they are 
quite smooth until subjected to the dissolving influence of an 
acid, when they acquire the same kind of rhombohedral ex- 
cavation on the surface that is seen in the spicules of Hyalo- 
nema Smithii &c. when found loose in the ‘ rotten” or dis- 
integrated rock. 
Lately Mr. Charles Moore, F.G.8., sent me some specimens 
of fossil sponge-spicules from the Liassic rocks of Brocastle, 
found also abundantly on the weathered surfaces of the Liassic 
rocks on both sides of the Bristol Channel, as a sample of 
Ann. & Mag. N. Hist. Ser. 5. Vol.i. 
