[dawsonI fossil sponges AND OTHER ORGANIC REMAINS 113 



spicules, and slender crueitnrni spicules avc likewise intei'niina;ied with the 

 rods in the strands. In the interstices of the mesh-work, and apparently 

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

 composed of slender rods and cruciform and other spicules, overlapping 

 each other without definite arrangement. It is prohable that this layer 

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

 chai'acter 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 probably 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 sj)onge, since the basal portion is wanting, and no anchoring- 

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

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

 liarlyornamented spiral ryds which nuiy belong to this species. 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 fibres. As with the other spicules, 

 these anchoi"ing-rods are now of pyrites. 



Later observations 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 that the strands of the framework have there an imperfect spiral 

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

 base they would asfume the form of the spiral 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 represents its entire skeleton. A similar condition seems 

 to have been present in ProtosjJOîigia, Cyathyphycus, and probably also 

 in Dictyospongia 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. 



