336 CORALS AND CORAL ISLANDS. 
observations of various voyages with those of the Expedition, 
the author has been enabled to draw these boundary lines 
with a considerable degree of accuracy, and they are laid 
down on the chart, Plate XVI., from the author’s Exploring 
Expedition Report on Crustacea. 
In the Pacific Ocean, this coral boundary, or isocryme of 
68°, passes near the Galapagos, but to the south of them, 
and thus approaches closely if it does not cross the equator, 
instead of being near the parallel of 28° south, its position 
in mid-ocean. Captain Fitzroy, R.N., found the surface tem- 
. perature of the sea at the Galapagos, from Sept. 16 to Oct. 
2» 18, 1855, 62° to 70° F., while, under the equator, about the 
middle of the Pacific, the range of surface temperature of the 
sea through the year is 81° to 88° F. 
Change of level would make great changes in the boun- 
dary. Mr. A. Agassiz found modern coral limestone near 
Tilibiche, Peru, at an elevation of two to three thousand feet ; 
showing that, in the early part of the coral-reef era, the con- 
tinent on the Pacific side stood this much lower than now, 
and that then corals were growing on part of its Pacific 
border. 
On the side of Asia the boundary line bends far south- 
ward, and reaches the coast of Cochin China within 15° cf 
the equator, although 50° from the equator a little to the 
eastward. On the west side of the Atlantic, the northern line 
starts at Cape Florida, in latitude 15° N., stretches abruptly 
northward, and bends around the Bermudas in latitude 33° N. 
On the African coast opposite, the northern line curves down- 
ward to the latitude of the Cape Verdes, and the southern 
upward nearly to the equator. The following table gives the 
positions of the coral boundary lines where they meet the 
coasts of the continents. 
