ZOOLOGY: A. G. MAYOR 393 
• 
These results account for the disappearance of the lithothamnion ridge 
as the reef grows outward. This ridge is formed only on the breaker-washed 
seaward edge of the reef and it projects somewhat above low tide level, but 
as the reef extends seaward the ridge disappears over the shoreward parts of 
the reef -flat. 
The growth-rate of these Samoan corals is nearly twice as rapid as is that of 
similar genera in Florida and the Bahamas, the growth rate of Atlantic corals 
being well determined due to the exhaustive studies of Vaughan. 
This is probably due to the fact that in the Atlantic the corals live chiefly on 
the outer edges of shallow limestone flats the muddy bottoms of which become 
churned up by the waves, and pelagic life is largely killed in the water 
over the flats. Thus in the Atlantic, the corals are well fed only on the in- 
coming tide, very little reaching them from the water passing outward from 
the flats. 
In the Pacific, on the other hand, the lagoons are generally deeper than in 
the Atlantic and there is but little limestone mud so that the corals receive 
food both from the rising and from the ebbing tide; and, as is well known, 
the growth-rate of coelenterates is a factor of their food supply. 
The following table shows the average increase in dimensions of corais from 
Samoa, and from the Florida-Bahama region; the growth-rate of the At- 
lantic genera being taken from the report by T. W. Vaughan, 1915, Year 
Book of the Carnegie Institution of Washington, No. 14, pp. 227-228. 
GENUS OF CORAL, AND LOCALITY 
AVERAGE 
INCREASE IN 
DIAMETER 
NUMBER OF 
RECORDS 
AVERAGE 
INCREASE IN 
HEIGHT 
NUMBER OF 
RECORDS 
mm. 
mm. 
Acropora from Samoa 
98 
21 
48 
21 
From the Atlantic according to Vaughan* 
54 
71 
35 
42 
Branched Po rites from Samoa 
47 
6 
25 
6 
From the Atlantic according to Vaughan 
26 
207 
19 
106 
35 
9 
From the Atlantic according to Vaughan 
13 
120 
* Vaughan 's estimate is based upon branching species such as Acropora cervicornis; A. 
prolifera and A. palmata. My records of somewhat similar species in Samoa show an aver- 
age increase in width of 178 mm. and 79 mm. in height. Many of the Samoan Acroporas 
are low, encrusting or dome-shaped forms such as are not found in the Atlantic. 
It will be observed that on the average the 21 Samoan Acroporas each 
gained 539 grams in weight per annum. This appears to be about 3 times 
as great as the annual increase in weight of Acropora palmata, of the Bahamas, 
for Vaughan 1915 loc. cit. p. 230 gives this as 173 grams per annum. 
The final paper of which this is a preliminary abstract will be published 
' by the Carnegie Institution of Washington. 
