338 On Coral Reefs and Islands. 
except the fith segment, which is silvery white. Thorax green- 
ish yellow. ‘Tail yellowish, margined with brown. Antenna, 
brown, attenuated, terminal hook small and acute. Legs, under 
surface of the head and thorax white, under surface of the abdo- 
men steel-gray, and of the tail yellowish green. Length 1,4, 
inch. Alar expansion 2,4, inches. 
veral specimens were taken at Poland, Mahoning Co., Ohio, 
eighteen years since ; one only has been preserved in my cabinet, 
and is somewhat impaired by fading and mutilation. Still, it as 
sufficiently perfect to allow of correct description. 
The perfect insects were observed flying at midday, and re- 
sembled in their habits the lady birds (Sesia), from which they 
differ in having opaque wings. apt \ 
- Harris did not embrace this species in his valuable descrip- 
tive list of Sphinges of the United States, in the 36th volume of 
the Journal of Science, and he informs us that it has not been 
described but resembles somewhat the M. zonata of the West 
Indies, but is larger and differently marked, especially upon the 
anterior wings. It will occupy a place in his arrangement imme- 
diately before the genus Sesia under the family Macroglossiade. 
Arr. XXX.—On Coral Reefs and Islands ; by James D. Dana. 
art Seventh. ; 
5. Geographical Distribution of Coral Reefs and Islands. 
Tue distribution of coral reefs over the globe depends on the 
following circumstances, arising from the habitudes of polyps 
already explained : 
1. The temperature of the ocean. 
2. The character of coasts as regards (a) the depth of water, 
—{b) the nature of the shores,—(c) the presence of streams. ae 
R = Liability to exposure to destructive agents, such as volcanic 
oi 
It has been stated that reef-growing corals* will flourish in the 
hottest seas of the equator, and over the ocean wherever the 
Winter temperature is not below 66° F. ’ The isocheimal line of 
this temperature therefore forms the boundary line of the coral- 
reef seas. 
This line traverses the oceans between the parallels 26° and 
30°, or in general near 28°. But in the vicinity of the conti 
