[DAWSON] FOSSIL SPONGES AND OTHER ORGANIC REMAINS 



113 





spicules, and Hleiidor criUMlorm spicules are likewise intenninified witli the 

 rods in the strands. In the iulerstiees of the mesli-work. and ap])areiitly 

 exterior to it, there is, iu some portions of the sponge, a thin open layer, 

 conipo.sed of slender rods and erucifonu and other spicules, overlappin<^ 

 each other without definite arranifenient. It is ])rol)al)le that tiiis layer 

 formed the outer surface of tiie sponge, for the spicules are of the .same 

 character as those of the strands of the mesh. 



The manner in which the small spicules seem to have drifted to one 

 side, shows that they proliahly formed the cortical layer covering the 

 whole surface, but became disengaged on decay of the soft parts. 



Some uncertainty arises respecting the anchoring appendages of 

 the sponge, sinee the basal portion is wanting, and no anchoring- 

 spicules are found in immediate ctmtact with the specimen, but on the 

 surface of the same rock-beds in which it occurs there are many pecu- 

 liarly ornamented spiral roils which may belong to this sj)ecie8. They 

 appear as if they consisted of several very minute filaments spirally 

 twisted together, like the strands of a rope. Each filament has a row of 

 projecting tubercles, which in the rod are definitely arranged in quin- 

 cunx, so that the general ornamentation is very striking. At the distal 

 end the rods are slightly curved, and the raised lines are more straight, 

 and assume more the aspect of distinct filu-es. As with the other spicu es, 

 these anchoring-rods are now of pyrites. 



Later observatiims tend to connect these anchoring-rods almost cer- 

 tainly with the present species. The rods are found almost exclusively 

 on the same surfaces with this sponge. They do not appear to belong to 

 any other form in these beds. Fragments of the base of the sponge 

 show thai the strands of the framework have there an imperfect spiral 

 arrangement, though slender, and if several of them coalesced at the 

 base they would assume the form of the spii-al rods. 



So far as can be at present determined this sponge appears to have 

 consisted simply of a delicate thin sack or cylinder of spicular strands 

 forming a rhombic mesh, with a thin outer spicular layer. There is no 

 evidence that the sack inclosed an inner spicular tissue, and probably 

 this thin wall repi'csents its entire skeleton. A similar condition seems 

 to have been present in Protosjwiujia, Cyathyphyrus, and probably also 

 in Dictyosponyia and the genera allied to it, but in none of these do we 

 find the structure on such a large scale as in the present form. 



