96 CORAL FORMATIONS. 
rature is above this, even in the hottest parts of the torrid zone, coral 
zoophytes thrive well. An isothermal line, crossing the ocean where 
this winter-temperature of the sea is experienced, one north of the 
line, and another south, bending in its course by divergence or con- 
vergence, wherever the marine currents change its position, will in- 
clude all the growing reefs of the world; and the area of waters may be 
properly called the coral-reef seas. This limiting temperature is found 
near latitude 28°. Under the equator in the Pacific, the waters where 
warmest, have the temperature 85° I’., and in the Atlantic 83° F.; 
66° to 85° is therefore not too great a range of temperature for the 
various reef-forming corals. Particular species, however, have smaller 
limits; but these limits have not yet been accurately ascertained.* 
The Porites and Pocillopore predominate at Oahu, (Sandwich 
Islands, ) and there are but few of the Astraide,—a fact which appears 
to be explained on the ground that the reefs of that island are not far 
from the cold limits of the coral seas: and it is interesting to observe 
that these same corals are the hardiest under exposures to impure 
waters. ‘The warmest parts of the ocean are favourable to the growth 
of Astras, Meandrinas, and the allied species; and at the same time, 
these regions abound in Porites and Pocillopore, although the propor- 
tion of these corals is smaller than at Oahu. 
The genera of reef-forming corals which occur out of the coral-reef 
seas, belong almost exclusively to the Caryophyllia family, and 
especially to the genera Dendrophyllia, Caryophyllia, Astroides,t 
Oculina, and Cyathina, some species of which exist in the Norwegian 
seas. ‘The Gorgonide, Aleyonide, Hydroidea, and Actinide, extend 
from the equator nearly to the frigid zone. The Bryozoa have an 
equally wide range.t 
The liability of the lagoons, when contracted in size, to become 
* The first application of the well-established principle that temperature influences the 
growth and distribution of corals is claimed by Mr. J. P. Couthouy equally with myself. 
Any attempt, however, to determine a limiting temperature he disclaims, and in this par- 
ticular, as well as the conclusions arrived at, our views are very different. The facts 
and inferences stated in this place, and on a following page, are deduced throughout 
from my own study and investigation. 
+ The corals of the Astroides closely resemble those of the Astraeze, and have been re- 
ferred to the latter group by many authors. A related species is found on the coast of 
this country as high up as lat. 42°. An Astrea has been reported from Sydney, New 
South Wales, which, if a true Astreea, (it has not been described or figured,) gives this 
genus a wider limit than the coral-reef seas. 
t See farther, Report on Zoophytes, p. 102. 
