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



A general difference must be pointed out in the fact that 

 Acrochæt. Dufoiirii seems to be somewhat smaller in all parts of 

 its thallus. 



The base in both plants appears to be very alike ; in both it 

 is a somewhat flat, upwards convex, below nearly plane disc, 

 formed by secretion of the basal cell; it is smaller in Acr. 

 Dufourii than that found in my plant, but in one specimen it 

 reached a diameter of about 14//. In Acr. Dufourii 

 the erect filaments are about 4 — bfi broad, in my 

 plant the filaments at their base are mostly 5 — 6 

 but specimens occur in which the filaments are S/u 

 thick. But while, and this is the most essen- 

 tial difference, the plant of Mr. Collins does not 

 Fig. 10. Aero- taper towards its summit the filaments in my plant 

 chætium Sar- taper very much and run out in thin nearly 

 spec. Branch colourless hair-like prolongations. This is not men- 

 with branch- tioned in the description of Collins and I have not 

 antheridia been able to find any trace of this in the dried 

 (About specimen in "Phycotheca". Besides, as is evident 

 from the above description, my plant is much more 

 branched and generally more developed (e. g. with sex organs) 

 than Acr. Dufourii. 



This species occurred together with several other upon old 

 leaves of Sargassum vulgare. 

 St. Thomas: The Harbour. 



2. Acrochætiiira crassipes Børgs. 



Børgesen, F., Some new or little known West Indian Florideæ, I. 

 (Botanisk Tidsskrift, vol. 30, 1909, p. 1). 



var. ty pica nov. var. 

 Børgesen, F., 1. c. 



My previous description of this plant was based upon few 

 specimens only ; now by renewed efforts I have been fortunate 

 enough to find it in other collections and this enables me to give 

 some additions to my former description. 



The basal cell is subcylindric-barrelshaped with thick walls 

 (Fig. 11 A, 5); in my former description I said that it was fastened 

 to the surface of the host plant by means of. a rather thick 

 layer of cementing substance and this is also the case where it 

 is growing e. g. on the spines of Centroceras as shown in the 

 fig. 11 .4 and B, but when found upon a more soft substratum 

 e. g. upon Hypnea it sometimes, at any rate, is somewhat im- 



