62 ON CORAL REEFS AND ISLANDS. 



The Porites and Pocilloporse predominate at Oahu, (Sandwich 

 Islands,) and there are but few of the Astrasidse, — 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 inter- 

 esting to observe that these same corals are the hardiest under 

 exposure to impure waters. The warmest parts of the ocean 

 are favorable to the growth of Astrseas, Meandrinas, and the allied 

 species; and at the same time, these regions abound in Porites 

 and Pocilloporae, although the proportion 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 fam- 

 ily, and especially to the genera Dendrophyllia, Caryophyllia, 

 Astroides,* Oculina, and Cyathina, some species of which exist 

 in the Norwegian seas. The Gorgonidse, Alcyonidas, Hydroidea, 

 and Actinidse, extend from the equator nearly to the frigid zone. 

 The Bryozoa have an equally wide range. f 



The liability of the lagoons, when contracted in size, to be- 

 come highly heated by the sun, is probably one cause leading to 

 the depopulation of these internal waters. The temperature be- 

 comes raised, as in a puddle of standing water elsewhere, and is 

 quite unfitted, therefore, for species accustomed only to the ordi- 

 nary tropical temperature of the ocean. 



Light and pressure and probably the amount of air in sea- 

 water, influence the growth of corals, so far as to fix limits to 

 their distribution in depth. It is a little remarkable that those 

 families which have a wide geographical range, have also a great 

 range in depth : for Caryophylliae, Dendrophylliae, Oculinae, Gor- 

 gonidae, and Hydroidea, are found even at depths of one or two 

 hundred fathoms; while Madrepores and Astrseas, and all the 

 ordinary reef-forming species scarcely exceed a depth of twenty 

 fathoms. 



Temperature has little or no influence in determining this 

 range, although it has been so asserted : 66° is not met with un- 

 der the equator short of 75 or 100 fathoms. The following table 

 gives approximate results for the winter months, from observa- 

 tions on this point by different navigators in the Pacific. It is 

 well known that these averages are varied much in particular 

 regions by currents. 



Latitude. Depth of 6° Fahrenheit. 



N. Latitude. 28°— 30' - - 0—25 fathoms. 



25° - - 25—30 



20° - - 30—50 



* The corals of the Astroides closely resemble those of the Astraea, and have 

 been referred 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 Astraea has been reported from 

 Sydney, New South Wales, which, if a true Astraea, (it has not been described or 

 figured,) gives this genus a wider limit than the coral reef seas. 



•j- See farther, Report on Zoophytes, p. 102. 



