60 James D. Dana, Esq., on the Structure, 



Some species of Nullipore, however, occur in these regions, 

 and others are found at various depths. 



As the Caryophyllia family extend into deeper waters than 

 most other reef corals, it might be inferred that these at 

 least may constitute a lower bed, or substratum. But this 

 is by no means the case. As just stated, one species (the 

 Dendrophyllia nigrescens) was found at 14^ fathoms, and 

 also of identical characters at low tide level. The Caryo- 

 phyllias are but sparingly distributed ; the species are few, 

 and mostly small, and not a dozen different kinds were de- 

 tected in the Pacific. Their contributions to reefs are there- 

 fore inconsiderable. 



4. Rate of Growth of Zoophytes. 



The rate of growth of zoophytes is a subject but little un- 

 derstood. We do not refer here to the progress of a reef in 

 formation, which is another question complicated by many 

 co-operating causes ; but simply to the rapidity with which 

 particular species of coral zoophytes increase in size. There 

 is no doubt that the rate is different for different species. It 

 is moreover probable that it corresponds with the rate of 

 growth of other allied polyps that do not secrete lime. The 

 rate of growth of Actiniae might give us an approximation 

 to the rate of growth in a Mussa, which are coral animals 

 of like size and general characters ; for the additional 

 function of secreting lime would not retard necessarily the 

 maturing of the polyp ; and from the rate of growth of the 

 same animals in the young state we might perhaps draw 

 some inferences as to the rate in polyps of corresponding 

 size. But no observations on this point were made by us 

 while abroad. 



Although the rapidity is undoubtedly far less than was 

 formerly reported, the following facts from different sources, 

 seem to shew that the rate is still greater than has been of 

 late believed. Mr Darwin, citing from a manuscript by Dr 

 Allen of Forres, some experiments made on the east coast of 

 Madagascar, states, that in December 1830 twenty corals 

 were placed by this gentleman apart on a sandbank, in three 

 feet water (low tide), and in the July following each had 



