Carter — Fossil Sponge- Spicules from Ben Bulben. 193 
To these may be added a sausage-shaped spicule like that of some of the 
Renierce of the present day, also tolerably plentiful (fig. 14) ; and other fusi- 
form acerate ones (figs. 15 and 16), which being common to many kinds of 
sponges, cannot in their isolated state, be identified with any in particular. 
Two fragments represent the arms of a quadriradiate spicule (fig. 17) ; but 
whether these were equal in length, or one was prolonged into a shaft, there 
is no evidence to show : if the former, it probably belonged to one of the 
Pachastrellina ; if the latter, to a zone-spicule of one of the Pachytragida. 
The most interesting part of this discovery, however, is that the “clay ” of 
Ben Bulben, in which Mr. Wright found these remains, is apparently identical 
in every respect with that sent me by Mr. James Thomson, in which he found 
Holasterella conferta , near Glasgow. In both instances isolated sponge-spicules 
of different kinds are disseminated through it, which can be obtained by 
edulc oration with water, and are composed of silica in an opaque or chalcedonic 
state, rendered more or less irregular by the presence of rhomboidal excava- 
tions on the surface. 
Here I might observe that, not only are the sponge-spicules, and the minute 
fossils of the Carboniferous Limestone which accompany them, silicified and 
pitted on the surface with the same kind of rhomboidal excavations, but the 
“chert’’ to which Mr. Wright has alluded appears to be a solid pseudomorph 
of the limestone ; for its pumice-like worm-eaten character occurring here 
and there, from partial absorption or decomposition of the material, presents a 
skeletal rhomboidal structure ; while the same kind of rhomboidal excavations 
characterize the surface of the weather-worn calcareous fossils in the pure 
Devonian Limestone of this neighbourhood ; by which I am led to infer that, 
in the first place, the sponge-spicules become partially or wholly calcified 
among calcareous material, else why should they now present rhomboidal exca- 
vations on their surface? that subsequently the siliceous element, being 
liberated, replaced the calcareous material so as to form the “chert;” and, 
thirdly, that the rhomboidal excavations on the surface of the spicules and the 
partial absorption of the spicules themselves, leaving nothing but their moulds, 
arises from the changes which the siliceous element itself is now undergoing— 
that is, becoming decomposed and removed, or passing from an amorphous 
state into clear quartz prisms. The latter, although but slightly the case, 
comparatively, in the specimens from Ben Bulben, is characteristically so in 
the specimens to which I have alluded from Black Head, Co. Clare, wherein 
not only geodic cavities lined with quartz prisms, but perfect prisms themselves 
are present, imbedded in the amorphous siliceous material composing the rock, 
■while all satisfactory traces of sponge-spicule form in these parts is entirely 
absent, so far as the specimens sent to me indicated. 
Lastly, I am inclined to think that the “ clay” of Ben Bulben is the “ chert” 
decomposed, and that the innumerable fragments of sponge-spicules which are 
present in the latter (for in some parts the chert appears te be almost entirely 
