AGE OF CORAL REEFS. 191 



each other as in the Brain-Corals, but in which the 

 depression formed by the mouths are deeper, 

 and the Caryophyllians, in which the single in- 

 dividuals stand out more distinctly from the 

 stock ; among Porites-, the P. Astrseoides, with 

 pits resembling those of the Astrseans in form, 

 though smaller in size, and growing also in solid 

 heads, though these masses are covered with 

 club-shaped protrusions, instead of presenting a 

 smooth, even surface like the Astraeans, and 

 the P. Clavaria, in which the stocks are divided 

 in short, stumpy branches, with club-shaped ends, 

 instead of growing in close, compact heads ; 

 among the Maeandrinas we should have the 

 round heads we know as Brain-Corals, with their 

 wavy lines over the surface, and the Manicina, 

 differing again from the preceding by certain de- 

 tails of structure ; among the Madrepodes we 

 should have the Madrepora prolifera, with its 

 small, short branches, broken up by very frequent 

 ramifications, the M. cervicornis, with longer 

 and stouter branches and less frequent ramifica- 

 tions, arid the cup-like M. palmata, resembling 

 an open sponge in form. Every Species, in 

 short, that lives upon the present Reef is found 

 in the more ancient ones. They all belong to 

 our own geological period, and we cannot, upon 

 the evidence before us, estimate its duration at 

 less than seventy thousand years, during which 



