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. Astreoides, with 
pits resembling those of the Astreans 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 Astreans, — 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 Meandrinas 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, and 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 
