162 J. D. Dana on Zoophytes. 
The range of these reef-forming corals in depth is singularly 
small. ‘T'wenty or perhaps sixteen fathoms will include very 
nearly all the species of the Madrepore and Astrea tribes.* Tem- 
perature has little or no influence in occasioning this limit, as 68° 
F, will not be found under the equator short of a depth of one 
hundred fathoms. Light and pressure, the latter affecting the 
amount of air for aération, are probably the principal causes. 
The waves, moreover, seldom reaching to a greater depth than 
thirty fathoms, cannot aid in renewing the, expended air below, 
as they do at the surface. 
In recapitulation we state that the Astreeacea, Madreporacea, 
and the miporide among the Caryophyllacea, are, with few 
exceptions, confined to the coral-reef seas,+ and to within twenty 
fathoms of the surface. The Caryophyllidet extend from the 
equator to the frigid zone, and some species occur at a depth of 
two hundred fathoms or more. The Alcyonaria have an equally 
wide range with the Caryophyllide, and probably reach still far- 
ther towards the poles. The Hydroidea range from the equator 
to the polar regions, but are most abundant in the waters of the 
temperate zone. 
Besides the above-mentioned limiting causes, there are others 
of importance, one of which may be alluded to in this place ; 
the remaining, belonging more properly to the Geological Report 
on Coral Reefs and Islands, will be particularly considered in the 
forthcoming volume by the author. The cause referred to, is that 
proceeding from original sites or centres of distribution. There is 
sufficient evidence that such centres of distribution, as regards 
zoophytes, are to be recognized. I'he species of corals in the 
West Indies are, in many respects, peculiar, and not one can with 
certainty be identified with any of the East Indies. he central 
parts of the Pacific Ocean appear to be almost as peculiar in the 
corals they afford. But few from the Feejees have been found 
to be identical with those of the Indian Ocean. A more com- 
plete acquaintance with the corals of these different seas, will, 
undoubtedly, multiply the number of identical species ; but ob- 
servations, thus far made, seem sufficient to establish as a fact 
that a large part of zoophytes are confined to a small longitudinal 
range. This will be seen from the following table, exhibiting, 
specified at top. 
- 
* The evidences on this point will be presented in the Report on Coral Islands. 
_ + The exceptions belong mostly to the genus Enuphyllia, which includes the 
genus Hum, some Turbinali#, and the Lobophyllia, having entire lamelle. 
+ The Caryophyllie of Blainville, with the Dendrophyllie, Oculine, &c 
