SPONGIDA. 



137 



rate, as a group of Calcareous Sponges with a continuous 

 skeleton thus giving us a link between the Spongida and the 

 Foraminifera. Of the fossils referred to, we need only speak 

 more particularly here of the abundant Palaeozoic forms which 

 have usually been grouped together by palaeontologists under 

 the name of Stromatopora, and of the singular forms com- 

 posing the genus Archceocyathus. The genus Stromatopora 

 using this term in a purely general sense comprises a great 

 number of Silurian and Devonian (possibly also Carboniferous) 

 fossils, which form hemispherical, globular, or irregular masses, 

 varying from an inch or less up to many feet in diameter, 

 and which are always composed essentially of closely ap- 

 proximated calcareous laminae (fig. 34) arranged concentri- 

 cally round one or more centres, and often demonstrably 



Fig. 34. A small and perfect specimen of Stromatopora rugosa (Hall). From the 

 Memoirs of the Geological Survey of Canada. 



attached to foreign bodies. Sometimes they form thinner 

 or thicker crusts, growing parasitically on shells or corals, 

 or spreading out as horizontal expansions. The concentric 

 laminae, which are the essential feature of this group of fossils 

 (and which strongly call to mind the appearance of masses 

 of Eozoori), are separated by wider or narrower interspaces, 

 which are more or less completely subdivided by vertical 



