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 otber 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 tbese 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 

 edulcoration 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 



