THE LOWER SILURIAN PERIOD. 99 



of the fossil a lattice-like appearance. There are also usually 

 minute pores in the concentric laminae, by which the successive 



Fig. 38. — A small and perfect specimen oi S tromatopora rugosa, of the natural 

 size, from the Trenton Limestone of Canada. (After Billings.) 



interspaces are placed in communication ; and sometimes the 

 surface presents large rounded openings, which appear to corre- 

 spond with the water-canals of the Sponges. Upon the whole, 

 though presenting some curious affinities to the calcareous 

 Sponges, Stromatopora is perhaps more properly regarded as 

 a gigantic Forami7iifer. If this view be correct, it is of special 

 interest as being probably the nearest ally of Eozoon^ the 

 general appearance of the two being strikingly similar, though 

 their minute structure is not at all the same. Lastly, in the 

 fossils known as Receptaculites and Ischadites we are also pre- 

 sented with certain singular Lower Silurian Protozoans, which 

 may with great probability be regarded as gigantic Foranii- 

 nifera. Their structure is very complex ; but fragments are 

 easily recognised by the fact that the exterior is covered with 

 numerous rhom.boidal calcareous plates, closely fitting together, 

 and arranged in peculiar intersecting curves, presenting very 

 much the appearance of the engine-turned case of a watch. 



Passing next to the sub-kingdom of Cxlenterate animals 

 (Zoophytes, Corals, &c.), we find that this great group, almost 

 or wholly absent in the Cambrian, is represented in Lower 



