54 James D. Dana, Estj., on the Structure^ 



is evidently the temperature congenial to them. From a 

 general survey of facts, it appears that these species are not 

 met with where the winter temperature remains much time 

 below 66° Fahr., though a temporary reduction to 64°, or 

 perhaps lower (as the Bermudas), may sometimes occur. 

 Wliere the temperature 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 tem- 

 perature of the sea is experienced, one north of the equator, 

 and another south, bending in its course by divergence or 

 convergence, wherever the marine currents change its posi- 

 tion, will include 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° Fahr., and in the Atlantic 83° Fahr. ; 

 ^^'^ to 85° is therefore not too great a range of temperature 

 for the various reef-forming corals. Particular species, how- 

 ever, have similar limits ; but these limits haVe not yet been 

 accurately ascertained.* 



The Porites and Pocilloporse predominate at Oahu (Sand- 

 wich Islands), and there are but few of the Astraeidse, — 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 exposure to impure w^aters. 

 The warmest parts of the ocean are favourable to the growth 

 of Astrseas, Meandrinas, and the allied species ; and at the 

 same time, these regions abound in Porites and Pocilloporse, 

 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 

 family, and especially to the genera Dendrophyllia, Caryo- 



* 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 tempe- 

 rature he declaims, and in this particular, 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. 



