DEPARTMENT OF MARINE BIOLOGY. 175 



the outer wall bears few other living corals. Frequently large areas, barren of 

 any living coral, were found to be surfaced with a layer of limestone from 3 

 to 18 inches in thickness, formed from the spicules of this species. Large 

 areas of this material were also found underlying the raised lithothamnium 

 ridge at the outer border of the reef. On the horizontal reef surface it is found 

 abundantly only in locations removed from the heavy surf, but where there is 

 still vigorous wave-action when the tide is low — a set of conditions found only 

 on the outer portion of some of the less-exposed reefs. 



A. confertum forms a nearly continuous carpet over large areas, often several 

 acres in extent, on the horizontal surface of many of the more protected reefs. 

 In such locations practically the entire reef surface for a depth of from a few 

 inches to more than 3 feet is composed of spicule rock, as the alcyonium has 

 crowded out all other species of coral. 



The two remaining species do not form incrusting colonies, but occur scat- 

 tered over the reef as circumscribed mushroom-shaped {A.glaucum) or shrubby 

 (A. flexile) growths. 



Several lines of squares, each 25 feet on a side, were laid out across different 

 reefs, and the part of each square occupied by Alcyonaria was determined. 

 Along these lines the Alcyonaria began from 50 feet to 400 feet from shore, 

 depending upon the topography of the shore-line, and extended to the inner 

 border of the lithothamnium ridge (about 50 feet from the edge of the reef), 

 forming an almost continuous carpet over a considerable portion of the line. 



Whether or not the Alcyonaria have in the past been as important limestone- 

 forming agents on the Samoan reefs as they now are can be determined only 

 after the examination of cores obtained from borings through the reefs. This 

 part of the work, as well as the final measurements of several hundred speci- 

 mens of Alcyonaria of all sizes to determine their growth-rate, must be com- 

 pleted at a later visit to this region. 



That the conditions found on the reefs at Tutuila are not exceptional is 

 shown by some observations of Dr. Carl Elschner, of Berkeley, California, who 

 recently sent to me for determination specimens of two species of Alcyonium 

 from Fanning Island, with the statement that this organism "forms fleshy and 

 slippery masses covering a great part of the lagoon bottom," and that at least 

 one-third of the surface rock at the bottom of the lagoon was composed of 

 spicules of Alcyonaria. 



Since this material is very rich in magnesium, as is also the coral laid down 

 by the coralline algae, it is quite possible that in some of the earlier determi- 

 nations, where chemical analysis was relied upon for the identification of 

 materials, the presence of limestone formed from these spicules has not been 

 recognized for its full value. 



Studies of Alcyonaria at Tortugas, by L. R. Gary. 



Measurements and photographic records of the growth-rates of several 

 species of Alcyonaria were continued from previous years. All of these speci- 

 mens were transferred last season from their previous locations to the surface 

 of a reef in 12 feet of water to insure uniform environmental conditions, since 

 it had been observed that the average size of mature colonies was greater in 

 deep than in shallow water. The results obtained this season indicate that 

 in the deeper water the growing-period is extended, as several specimens 

 which had been practically at a standstill for 4 years while on shallow reefs 

 showed a measurable increase in size during the past year. 



The resistance to heat in stony corals has been found by Mayer,^ to corre- 

 spond in general to their distribution on the reefs. Those forms which are 



'Carnegie Inst. Wash. Pub. No. 183. 



