26 



most often only few occur in the rhizines, above these a side- 

 branch sometimes grows out. 



The chromatophores are small polygonal plates (mostly 

 trigonal) with prolonged edges and in the young cells forming 

 a net work; in older cells the chromatophores are mostly isolated; 

 each chromatophore contains a pyrenoid. 



As to the cell-division, I have not been able to see in my 

 material with certainty how it is accomplished but most probably 

 the walls are formed in a similar way as in Cladophora; lenti- 

 form cells are wanting. I have never found ball-formation in 

 this form. 



The small cells filling up the intervals between the large 

 cells of the radiating branch-system have at their apices a ring- 

 formed thickness by means of which they are firmly attached 

 to the neighbour-cell; this thickened cellulose ring reminds very 

 "much of those found in Microdictyon. 



This species occurs both in shallow water and in deep sea 

 down to a depth of about 50 meters. In shallow water it is found 

 in rather exposed places growing in fissures in rocks. The speci- 

 mens from deep sea are often large, reaching a length of 4 cm. 

 or more. 



It is found: St. Croix; Cane Bay; St. Thomas: The Harbour, the 

 sound between Water Island and St. Thomas; St. Jan: Off Cruz Bay, 

 America Hill, Rams Head., Hermitage and several other places. 



Geogr. Distrib. Mediterranean, West Indies, Brazil. 



Microdictyon Decsne. 



1. Microdictyon umbilicatum (Veil.) Zanard. 



Zanardim, Iconographia Phycologica Adriatica, vol. I, p. 79, tav. 

 XIX, 1860. Hauck, Meeresalgen, p. 467. 



Conferva umbilicata Velley in Transact. Linn. Soc, Vol. V. London 

 1800, p. 169, tab. VII. 



Hydrodictyon umbilicatum C. Ag., Systema, 1824, p. 85. 



Microdictyon Agardhianum Decsne, Plantes de I'Arabie heureuse (Ar- 

 chives du Museum, t. II, Paris 1841, p. 116). Collins, Green Algae, 

 p. 366. 



I have compared the West Indian plant with specimens I 

 have collected at Cadiz; both forms seem to agree very well, 

 the (Cadiz plants being somewhat more loosely and openly l)iiilt 

 (f. tenuis of C. Agardh 1. c), most probably due to the condition 

 of life, growing as they did in a sheltered bay among sea-grasses, 

 whilst the West Indian form though found in deep water was 

 growing in localities where strong cnrrenls prevail. 



