110 TENTH ANNUAL REPORT OF 



EASTERN SHORES OF NORTH AMERICA. 



In comparing the marine vegetation of the opposite shores of the 

 northern Atlantic, a great resemblance is observed between the ordi- 

 narj^ seaweeds that clothe the rochs on the eastern and western sides; 

 with this diiference, that the species do not reach so high a latitude 

 on the American shore as on the European. The reason of this will 

 be readily understood by inspecting a physical map of the Atlantic, on 

 which Humboldt'sisothermal lines, or lines of mean annual temperature, 

 are laid down. For then it will at once be seen that there is a very con- 

 siderable bending of the isothermal lines in favor of the continent of Eu- 

 rope. Thus the same line that runs through New York, in lat. 41°, 

 strikes the shores of Europe in the north of Ireland, lat. 54°, And 

 though there is less difference in mean temperature in the southern 

 parts of the continents than in the northern, still there is a marked 

 difference throughout. 



With respect to vegetation, Laminar ia longicruris is common on 

 the American shore — at least as far south as Cape Cod (lat. 42°) ; 

 while on the European it has not been found south of Norway, save 

 some stray, waterworn stems occasionally cast on the north of Ireland 

 or Scotland. 



Bodymenia crystata, so very abundant in Boston harbor, (42° 30') 

 where it enters largely into the composition of seaiveed pictures, is 

 rarely found in Europe south of Iceland and the northern parts of 

 Norway ; its most southern limit being in the Frith of Forth, (56°), 

 where it has been found but once or twice, 



Delesseria liypoglossum has not been observed in America north of 

 Charleston, (lat. 33°), while in Europe it occurs in Orkney, (lat. 59°), 

 and is in great profusion and luxuriance on the north coast of Ireland 

 in lat. 55°. The distribution of this species on the American shore 

 is ver}'- anomalous if Charleston be its northern limit, for it certainly 

 extends southward at least to Anastasia Island, (lat. 29° 50'). In 

 the British seas it is most luxuriant on the Antrim shore, (55°), where 

 its fronds are sometimes three feet in length ; southern specimens are 

 generally much smaller, and in Devonshire it rarely measures more 

 than three or four inches, which is the average size of specimens from 

 the south of Europe, as well as of those found in Charleston harbor. 

 If we are correct in limiting the American distribution of this species 

 northward by Charleston, we have the remarkable fact that the great- 

 est latitude attained by Del. hypoglossum in the northwestern Atlantic 

 is less by about 5° or 6° than the southern limit of the same species 

 on the northeastern, and by about 2*7° than the northern boundary of 

 its distribution. This indicates a range which the isothermal lines 

 can scarcely explain ; for the line which runs through Charleston 

 strikes the coast of Spain. It is the more remarkable in this species, 

 because the genus Delesseria is most numerous in the colder parts of the 

 sea, its finest species being natives of Northern Europe and of Cape 

 Horn and the Falkland Islands ; and, as we have seen, this very D. 

 hypoglossum is nowhere of greater size or in greater plenty than in 

 latitude 55° on the Irisli coast. 



