Mr. H. J. Carter on Fossil Sponge-spicules. 213 



raeter 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 exca- 

 vations on the surface of the spicules and the partial absorp- 

 tion 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 re- 

 moved, or passing from an amorphous state into clear quartz 

 prisms. The latter, although but slightly the case, compara- 

 tively, 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 geoclic cavities lined with quartz 

 prisms, but perfect prisms themselves are present, imbedded 

 in the amorphous siliceous material composing the rock, 

 Avhile 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 to be almost entirely 

 composed of them) , rendered still more fragmentary by par- 

 tial removal so as to leave nothing but their moulds, as before 

 stated, are those which at last come out entire, so far as they 

 go, in the washing of the " clay." 



It is remarkable, too, that by far the most plentiful among 

 Mr. Thomson's collection of spicules from the clay near 

 Glasgow is that of Holasterella conferta, as it is that of 

 H. Wrightii at Ben Bulben ; the " sausage-shaped " spicule 

 (fig. 14) is also analogous to that of the supposed Renierid 

 sponge ('Annals,' 1879, vol. iii. pi. xxi. fig. 11), and about 

 the same in frequency. In Mr. Thomson's collection were 

 also fragments of Lithistid spicules ; and last summer he sent 

 me a section of an entire sponge in Carboniferous Limestone, 

 all calcified, with weathered-out spicules on the surface, but 

 none of it sufficiently defined for useful delineation. The 

 collection also contained some zone-spicules of the Pachy- 

 tragida ; so that, altogether, the Spongida appear to have been 



