GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. I457 



of the polar regions ; but these should be classed with the animals of 

 the continents ; and the continental isotherms or isocrymes, rather 

 than the oceanic, are required for elucidating their distribution. 



It seems necessary to state here the authorities for some of the 

 more important positions in these lines, and we therefore run over the 

 observations, mentioning a few of most interest. There is less necessity 

 for many particulars with reference to the North Atlantic, as our 

 facts are mainly derived from Lieut. Maury's Chart, to which the 

 author would refer his readers. 



1. North Atlantic. — Isocryme of 74° F. — This isocryme passes near 

 the reefs of Key West, and terminates at the northeast cape of Yu- 

 catan ; it rises into a narrow flexure parallel with Florida along the 

 Gulf Stream, and then continues on between the Little and Great 

 Bahamas. To the eastward, near the African coast, it has a flexure 

 northward, arising from the hot waters along the coast of Guinea, 

 which reach in a slight current upward towards the Cape Verde 

 Islands. The line passes to the south of these islands, at which group, 

 Fitzroy, in January of 1852, found the sea-temperatures 71° and 

 72° F. 



Isocryme 0/68 F. — Cape Canaveral, in latitude 27° 30', just north of 

 the limit of coral reefs on the east coast of Florida, is the western 

 termination of the line of 68°. The Gulf Stream occasions a bend in 

 this line to 36° north, and the polar current, east of it, throws it 

 southward again as far as 29° north. Westward it inclines much to 

 the south, and terminates just south of Cape Verde, the eastern cape 

 of Africa. Sabine found a temperature of 64° to 65° F. off Goree, 

 below Cape Verde, January, 1822; and on February 9, 1822, he 

 obtained 66 i° near the Bissao shoals. These temperatures of the cold 

 season contrast strikingly with those of the warm season. Even in 

 May (1831), Beechey had a temperature of 86° off the mouth of Rio 

 Grande, between the parallels of 11° and 12° north. 



Isocryme of 62° F. — This isocryme leaves the American coast at 

 Cape Hatteras, in latitude 35 h° north, where a bend in the outline of 

 the continent prevents the southward extension of the polar currents 

 from flowing close along the shores. It passes near Madeira, and 

 bends southward reaching Africa nearly in the latitude of the Canaries. 



Isocrymes of 56° and 50° F. — Cape Hatteras, for a like reason, is 

 the limit of the isocrymes of 56° and 50° as well as of 62°, there being 



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