ALVEOLINA 



805 



especially ^abundant in those of some of the Philippine Islands, of the 

 Ked Sea. of the Mediterranean, and especially of the ^-Egean. When 

 such discs are subjected to microscopic examination, they are found 

 (if uninjured by abrasion) to present tin- ,-t nu-ture represented in 

 lit--. <><)i. where we set' on the surface (by incident light) a number 



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of rounded elevations, arranged in concentric zones around a sort of 

 nucleus (which has been laid open in the figure to show its internal 

 structure) ; whilst at the margin we observe a row of rounded pro- 

 jections with a single aperture or pore in each of the intervening 

 depressions. In very thin discs the structure may often be brought 

 into view by mounting them in Canada balsam and transmitting 



