102 CORAL FORMATIONS. 
the surface, and was quite immovable; and some had grown over the 
others. Mr. Stutchbury describes a specimen consisting of a species 
of oyster, whose age could not be over two years, which was encrusted 
by an Agaricia, weighing two pounds nine ounces.* It is stated by 
M. Duchassaing, in a letter from Guadeloupe, that in two months, some 
large individuals of the Madrepora prolifera, which he broke away, 
were restored to their original size. 
Since the return of the Expedition, I have received a letter con- 
taining some facts on the growth of Actinie from Sir J. G. Dalyell, 
whose able observations in this department of science are highly 
curious and important. After speaking of the various conditions and 
sizes of the young at birth, and of the difference in the rapidity of 
growth depending on the amount of nutriment at hand, he says, 
speaking of a Scottish species of Actinia, “The dimensions will 
generally double in a fortnight from its birth. The diameter of the 
base being originally about an eighth of an inch, or hardly as much, 
will be five-eighths in six months, and the tentacles will occupy a 
circle of an inch and a half in diameter. In twelve or thirteen months, 
the diameter of the base will reach an inch and the expansion of the 
tentacles two inches between the tips. An Actinia whose tentacula ex- 
panded a quarter of an inch three weeks after it was produced, enlarged 
so much in five months that they expanded an inch, and the body was 
then half aninch thick.” If we reason upon this data, and assume that 
the Madrepore polyps may increase lineally in six months as much as 
the young Actinia, we shall have an elongation of five-eighths or three- 
fourths of an inch in six months. ‘Taking the still more rapid rate, of 
doubling in a fortnight, which might be more correct, since the 
Madrepore polyps are about the size of the Actinia in its earliest 
state, we should have a lengthening of a fourth of an inch in a month, 
and three inches a year. The data upon which this conclusion is 
based, though important, are uncertain, but would probably give too 
high rather than too low an estimate. And yet it is far below the rate 
apparently established by the experiments with corals cited in the 
preceding,paragraph. We must admit that the subject requires more 
accurate investigation. 
The stay of the Expedition near any particular reef in the Pacific 
was too short for any examinations by us. They might easily be 
* West of England Journal, vol. i. p. 50. 
+ L’Institut, No. 639, April 1, 1846, p. 111. 
