GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. I459 



have the character rather of those of the British Channel than of the 

 Mediterranean. 



Isocryme of 44° F. — This line commences on the west, at Cape 

 Cod, where there is a remarkable transition in species, and a natural 

 boundary between the south and the north. The cold waters from 

 the north and the ice of the Newfoundland Banks, press the line close 

 upon those of 50° and 56° F. But after getting beyond these in- 

 fluences, it rapidly rises to the north, owing to the expansion of the 

 Gulf Stream in that direction, and forms a large fold between Britain 

 and Iceland; it then bends south again and curves around to the 

 west coast of Ireland. 



Isocryme of 35° F. — This line has a bend between Norway and Ice- 

 land like that of 44°, and from the same cause, — the influence of the 

 Gulf Stream. But its exact position in this part has not been ascer- 

 tained. 



2. South Atlantic. — Isocryme of 74° F. — This line begins just 

 south of Bahia, where Fitzroy found in August (the last winter 

 month) a temperature of 74° to 75 i° F. During the same month he 

 had 75i° to 76 i° F. at Pernambuco, five degrees to the north. Off 

 Bahia, the temperature was two degrees warmer than near the 

 coast, owing to the warm tropical current, which bends the isocryme 

 south to latitude 17° and 18°, and the cold waters that come up the 

 coast from the south. The line gradually rises northward, as it goes 

 west, and passes the equator on the meridian of Greenwich. Sabine, 

 in a route nearly straight from Ascension Island, in 8° south, to the 

 African coast under the equator, obtained in June (not the coldest 

 winter month) the temperatures 78°, 77°, 74°, 72-8°, 72-5°, 73°, the 

 temperature thus diminishing on approaching the coast, although at 

 the same time nearing the equator, and finally reaching it within a few 

 miles. These observations in June show that the isocryme of 74° F. 

 passes north of the equator. The temperatures mentioned in Maury's 

 Chart afford, the same conclusion, and lead to its position as laid down. 



Isocryme of 68° F.— On October 23d to 25th, 1834, Mr. D. J. 

 Browne, on board the U. S. Ship Erie, found the temperature of the 

 sea on entering the harbour of Rio Janeiro, 67 h° to 68£° F. Fitzroy, 

 on July 6, left the harbour w T ith the sea-temperature 70 i° F. Beechey, 

 in August, 1825, obtained the temperatures 68\L6° to 69*66° F. off 

 the harbour. The isocryme of 68° F. commences therefore near Rio, 

 not far south of this harbour. Eastward of the harbour, the tern- 



