488 Dansk Botanisk Arkiv, Bd. 3. Nr. 1. 



stones more or less abundantly^). In additions to Caulerpa 

 many other algæ are characteristic of such localities; of the 

 Chlorophyceæ we find for instance Dichjosphæria favulosa and 

 D. van Bosseæ, Valonia ventricosa, V. utriciilaris and V. ma- 

 crophysa all forming smaller or larger crust-like bodies, also Cla- 

 dophoropsis membranacea occurs fixed as well as detached form- 

 ing Ægagropila-like bodies upon the bottom. And these are joined 

 with Chamædoris annulata, Halimeda Opuntia and H. incrassata., 

 Penicillus and many others. Of Phæopkyceæ we most frequently 

 find several Padina- and Dictyota-sipecie&, tufts of Ectocarpus^ 

 Colpomenia and Hydroclathriis etc. and finally many Rhodo- 

 phyceæ, of which the most characteristic are Gelidiopsis rigida, 

 Bryothamnion trianguläre^ Digenea simplex^ Laurencia papulosa, 

 Hypnea- and Spyridia-iorms, Galaxaiira, Amphiroa fragilissima 

 and other corallinaceous algæ, Centroceras and many other species, 

 all forming larger or smaller tufts often intermingled between 

 each other. 



Upon the very exposed rocky coast of the northwest side 

 of St. Croix several algæ occur so high above the sea-level that 

 they are moistened only by the spray. Uppermost were found 

 several Myxophyceæ and small tufts of Laurencia ohtusa and 

 L. papulosa, of Dilophus guineensis, Dictyota ciliata and Padina 

 gymnospora. Amongst these, scattered tufts of Chætomorpha an- 

 tennina were found in abundance and lower down Polysiphonia 

 ferulacea, Centroceras clavulatum and Ectocarpus breviarticulatus. Ex- 

 tensive patches are formed by Aglaozonia canariensis, which covers 

 the very uneven rocks with its fleshy — cartilaginous red-brown 

 thallus; now and then Ralfsia expansa and Lithothamnion, too, 

 form smaller or larger crusts. This association can perhaps be 

 compared with that of Callithamnion arbuscula at the Færoes, 

 the tropical association like the northern one living in such ex- 

 posed places upon steep rocks where it is exposed to the whole 

 force of the sea, and some of the tropical algæ, too, being able 

 to retain the water like small sponges. 



Another characteristic association of algæ, corresponding to 

 the Bangia — f/ro^/jora-association of the Færoes was found in 

 rather exposed places upon the steep rocky coast at Cruz Bay, 



^) BöRGESEN, F., An ecolog. and system, account of tlie Caulerpas, 1. c. 

 p. 346. 



