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gion. The reason why he chooses this curve for the lower limit is, 

 that the algæ-vegetation at and below 20 fathoms becomes uniform 

 and poor in species, as the greater part of the subHttoral algæ 

 will not thrive at so great a depth. He himseif points out, that 

 this Hmit is »weniger scharf oder wenigstens nicht so auffallend« 

 between these regions as between the hitoral and the sublittoral. 

 In agreement with Hose uvinge (45, p. 2.37), I therefore think, 

 that it is unadvisable to determine the boundary line between 

 two regions so arbitrarily, as the number of species and the 

 luxuriancy of the vegetation are in reality reduced regularly and 

 gradually from a certain depth downward. Moreover the sublittoral 

 species growing at the lowest depth are likewise often found in 

 the upper part of the sublittoral region. No other species are in- 

 Iroduced here that might change the appearance of the vegetation. 

 When moving downwards from the surface of the sea, the brown 

 algæ first disappear, and at last hardly anything but Florideæ 

 are left^ 



Finally, the curve of 20 fathoms may serve as a boundary line 

 only in the northern seas, wdiere the light does not reach far down. 

 In the Mediterranean Sea, a vigorous vegetation is still found at a 

 depth of more than 40 fathoms, and in the Tropics we may be 

 sure to find a rich algæ-vegetation still farther down. It does not 

 seem to me natural therefore to maintain this boundary line, as the 

 algæ-vegetation is luxuriant down to a certain depth, (in the Færoes 

 to about 15 fathoms, as far as my observations go) and then leaves 

 off slowly and gradually, as the depth increases. When Kjellman 

 objects (45, p. 8 — 9) to Ørsted 's division of the algæ-vegetation in 

 the Sound, that the boundary line between the brown and the 

 red algæ is indistinct, it seems to me that the same objection might 

 be applied to Kjellman 's division into a sublittoral and elittoral 

 region. If the sublittoral region must be divided, it is probably 

 Ørsted 's division that ought to be maintained, as it would seem 



* In connectlon with this I shall refer to Rosenvinge's summaiy of the species 

 (71, p. 231) found at a depth of at least 10 fathoms; algæ found at a still greater depth, 

 at 15 fathoms and more, are printed in thick type. As pointed out b}' Rosen- 

 vinge (1. c. p. 232), it is evident from the list that the red algæ increase in number 

 with the depth. There is hardly any doubt, that here also an almost pure Florideæ- 

 formation will be found, shortly before the vegetation ceases. It is true that Rosen- 

 vinge in the same connection (p. 236) speaks of a Desmarestia aciileata, found at 

 a depth of about 30 fathoms, near Upernivik. But I doubt whether this alga was 

 adhering to any substratum. 



48* 



