[DAWSON] FOSSIL SPONGES AND OTHER ORGANIC REMAINS 99 



sometimes forming un external dermal coiitinj;-, were minute spicules and 

 delicate protective spines. The spicules, originally composed of amor- 

 phous or colloidal silica, are now for the most i)art entirely replaced by 

 pyrite, and not infrequently they are also encrusted with a delicate 

 ' coating of minute crystals of the same mMieral, so as greatly to increase 

 their apparent magnitude, though in most cases it is possible under the 

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

 appears 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 the material of the 

 spicules has been entirely removed, leaving their imjjressions merely on 

 the matrix. It sometimes happens, especially in the caso» of species with 

 somewhat dense spicular 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 situ, so that when the shale is 

 split they appear in transverse section or as round flattened discs ; but in 

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

 or without their anchoring-rods, and to have been flattened later- 

 ally. When entire, they sometimes present, when the shale is split 

 ojjcn, a surface of dermal spines, masking the skeleton proper. In other 

 cases the dermal spines come away with the matrix, lea\nng the skeleton 

 spicules exposed. Thus the same species may i:)resent very ditferent 

 appearances under ditferent circumstances. In most cases the body of 

 the sponge has been more or less disintegrated or reduced to patches of 

 loose spicules, and some large surfaces are 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. Hinde doubts 

 this, and regards them as mei-ely loose spicules drifted together into 

 hollows or Avorm-burrows. 



All these differences of preservation and exposure present consider- 

 able difficulties in discriminating the species ; and these are sometimes 

 increased by the association of specimens of diff'erent ages. It thus 

 requires experience and alnindant material to obtain definite results. 

 Nevertheless Dr. Hinde, 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 opportune from the fact that 

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



