SPONGIDA. 7 1 



Horny Sponges in our present seas, may be explained by the 

 _ fact that the members of this group cannot be recognised in a 

 ' fossil state except by their siliceous spicula, and that these are 

 only sometimes present, and are necessarily very difficult of 

 detection. The genus Cliona alone, comprising the singular 

 boring sponges, has managed to survive from the commence- 

 ment of the Palaeozoic period to the present day. Shells 

 mined by species of this genus occur in the Silurian Rocks, 

 and the genus is well represented in recent seas. 



FOSSILS OF DOUBTFUL AFFINITIES. Before leaving the Pro- 

 tozoa, there are two fossils of doubtful relationships which may 

 be briefly noticed viz., Stromatopora and Receptaculites. These 

 show points of affinity to the Foraminifera on the one hand, 

 and the Sponges on the other hand, whilst the former ap- 

 proaches the Corals in some respects. Both, however, may, 

 with the greatest probability, be regarded as peculiar types of 

 Sponges. 



Stromatopora (fig. 19) forms hemispherical, globular, or 

 irregular masses, often of very considerable size, and sometimes 



Fig. 19. A small and perfect specimen of Stromatopora. rugosa (Hall). 

 From the Memoirs of the Geological Survey of Canada. 



demonstrably attached to shells. In its structure, Stromato- 

 pora consists of numerous thin, concentric laminae, penetrated 

 by minute tubes, the mouths of which appear on the surface 



