[DAWSON] FOSSIL SPONGES AND OTHER ORGANIC REMAINS 



99 



sometimes forming an external dermal coatinu;, were minute spicules and 

 delicate ))rotective spines. The spicules, originally composed of amor- 

 ))hous 01- colloidal silica, are now for the most part entirely replaced by 

 pyrite, and not infreciuently they arc also encrusted with a delicate 

 coating of minute crystals of the same mineral, so as greatly to incieaso 

 their ap])arent magnitude, though in most cases it is possible under the 

 lens to distinguish the original spicule from its coating. The sponge thus 

 ajjpcai-s as a delicate bronze-coloured framework or mass of spicules on 

 the surfaces of the shale. In a few instances the spicules have retained 

 their primitive siliceous material, and more rarely tiie nniterial of the 

 spicules has l)ecn entirely removed, leaving their imjyressicms merely on 

 the matrix. It sometimes hap])ens, especially in the case of species with 

 somewhat dense 8picu;nr walls, that the meshes included in the spicular 

 framework are tilled in with pyrite, so as to show merely the general 

 form and faint indications of the spicular structure. 



Originally rooted in the soft ooze of the sea bottom, the specimens 

 seem sometimes to have been buried in xitu, so that when the shale is 

 s[)lit t''ey appear in transverse section or as round tlattened discs; but in 

 most cases they seem to have drifted from their anchorage, either with 

 or without their anchoriiig-rods, and to have lieen flattened later- 

 ally. When entire, they sometimes ])i'esent, when the shale is split 

 ojien, a surface of dermal spines, masking the skeleton ])ropei\ In other 

 cases the dermal s])ii\e8 come away with the matrix, leaving the skeleton 

 spicules exposed. Thus the same species may presen* very difll'erent 

 appearances under difl'erent circumstancos. In most cases the body of 

 the sponge has been more or less disinttgrated or reduced to ])atches of 

 loose spicules, and some large surfaces ai'e covered with a confused coat- 

 ing of spicules and anchoring-rods belonging to several species. In some 

 cases also the loose spicules, or fragments of them, seem to have been 

 gathered in little oval or cylindrical piles and inclosed in pyrite. At 

 first I was disposed to regard these as coprolitic ; but Dr. Ilinde doubts 

 this, and i-egards them as merely loose spicules drifted together into 

 hollows or worm-burrows. 



All these differences of preservation and ex|)osure ])resent consider- 

 able difficulties in discriminating the species; and these are scmietimes 

 increased by the associatitm of 8])ecimens of different ages. It thus 

 requires experience and abundant material to obtain definite I'esults. 

 Nevertheless Dr. Ilinde, who has had very extensive acquaintance with 

 fossil sponges in various conditions of preservation, makes the following 

 remarks in reference to the specimens submitted to him : 



•'The Metis specimens are specially interesting, since they throw 

 much fiesh light on the character of the earliest known forms of these 

 organisms, and their discovery is the more op])ort»nie from the fact that 

 our knowledge of the existing hexactinellid sponges — the group to which 



