[dawbon] fossil sponges AND OTHER ORGANIC REMAINS 99 



sometimes forming an externul dermal coating, were minute spicules and 

 (leiicate protective spines. The spicules, originally composed of amor- 

 phous or colloidal silica, are now for the most part entirely replaced by 

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

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

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

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

 a])peai"S as a delicate bronze-coloured frameAVork 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 impressions merely on 

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

 somewhat dense spicular walls, that the meshes included in the spicular 

 framework are filled 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 ; Init in 

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

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

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

 02)en, a surface of dermal spines, masking the skeleton proper. In other 

 cases the dermal spines come away with the matrix, leaving the skeleton 

 spicules exposed. Thus the same species may present very different 

 appearances under different circumstances. In most cases the body of 

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

 loose spicules, and some large surfaces are covered with a confused coiit- 

 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 ; l)ut Dr. liinde doubts 

 this, and regards them as merely loose spicules drifted together into 

 hollows or worm-burrows. 



All these differences of preservation and exposure pi-esent consider- 

 able difficulties in discriminating the species ; and these are sometimes 

 increased l»y the association of specimens of different ages. It thus 

 requires experience and abun'hmt material to obtain <lefinite 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 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 hexactinellid sponges — the group to which 



