246 COMPARATIVE ZOOLOGY. 
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Fig. 201.—Astrea rotulosa. West Indies. 
The Astrea, Meandrina, Madrepore, and Porites are 
the chief reef-forming corals. They will not live in 
waters the mean temperature of which is below 68° 
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magnified. The cup-like depres- Fra. 203.—Fragment of Red Coral (Coral- 
sion at the top of a coral skele- lium rubrum), showing living cortex 
Fia. 202.—Cell of Madrepore Coral, 
ton is called calicle. and expanded Polyps. Mediterranean. 
