ON FOSSIL SPONGES PEOM METIS. 35 



spicvilar patches or layers. lu other iiistauees, however, they remain approximately in 

 their natural position, and even the general outline of the form ean be observed. 



The following additional remarks as to the state of preservation and characters of 

 the specimens are from notes made by Dr. Gr. J. Hiude, F.G.S. : — 



" The Metis specimens are specially interesting, since they throw much fresh light 

 on the character of the earliest known forms of these organisms, and their discovery is 

 the more opportune from the fact that our knowledge of the existing hexactiuellid 

 sponges — the group to which all, or nearly all, these fossils belong — has been vastly 

 increased by the work of Prof F. E. Schulze, of Berlin, on the hexactiuelled sponges 

 dredged up by the Challenger Expedition, and thus we are now better enabled than 

 hitherto to compare the fossil and the recent forms. 



" In the present specimens, the amorphous or soluble silica, of which their spicular 

 skeletons were originally composed, has entirely disappeared, and the spicules now con- 

 sist of iron pyrites. This replacement by pyrites is of common occurrence, more parti- 

 cularly in a matrix of black shales ; for example, the earliest known sponge, Protospo/igia 

 feuestrata, Salter, from the Cambrian rocks of South Wales, is in the same mineral condi- 

 tion, and in a nearly similar matrix, as the specimens from the Quebec group and the 

 Utica shale. When thus replaced, the general outline of the larger spicules is fairly 

 distinct, but where the spicules are minute, and in close proximity to each other, their 

 individual outlines are blurred by the tendency of the crystals of the replacing pyrites 

 to amalgamate together so as to form a continuous film of the mineral in which the finer 

 spicular structures are quite indistinguishable. This coalescence of the pyrites likewise 

 makes it very dilficult to determine whether the spicular elements of the sponge were 

 organically soldered together into a siliceous mesh, or whether they were merely held in 

 their natural positions by the soft animal structures, and owe their present union to 

 subsequent fossilisation. 



" Next to the chemical changes, we have to take into accovTut those produced on the 

 original structures of these sponges by what may be termed the mechanical influences 

 of fossilisation. There can be no doubt that they were hollow sacci-form or_vasi-form 

 structures with very delicate walls of spicular tissue, supporting the soft animal mem- 

 branes. They existed at the sxrrface of the soft ooze of the sea-bottom, and their basal 

 portions were probably embedded in it. They were furnished with elongated spicules 

 whos(> extension into the mud served to anchor them in one spot. After the death of the 

 animal, and ihe decay of the soft tissues, the delicate skeletal framework would be grad- 

 ually buried in the accumulating sediments, until by their weight it became completely 

 flattened. Under favorable circumstances, the outline of the sponge and the natural 

 arrangement of the spicular skeleton would be preserved, and this is fortunately the 

 case with the specimens of Cyathophycus from the Utica shale, and with some of the speci- 

 mens of Protospongia from Metis. More frequently, however, probably owing to currents 

 and other causes acting at the surface of the ooze, the skeletal framework is partially or 

 wholly broken up, so that only small patches of the connected skeleton, or merely the 

 dislocated and detached spicules irregularly scattered over the rock surface, remain for 

 determination, and this is the present condition of the majority of the specimens from the 

 Quebec group. For some reason, probably connected with the arenaceous character of 

 the rock in which they occur, the nearly allied sponges belonging to the Devonian genus 



