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



by its much more robust habit, forming a dense tuft composed 

 in older plants of many more filaments arising from the basal 

 disc. The erect filaments are furthermore divided from near 

 their base. The chromatophore is vigorously developed, forming 

 irregularly shaped plates. The filaments are 7 — 10 pi thick, tapering 

 very slightly from the base upwards, their upper ends being 

 5 — 6// thick \Nith obtuse apices and with well developed chrom- 

 atophores, even in the upper cells. The sporangia are smaller, 

 about 9/i broad and 15// long. 



Finally Acr. Hoytii, according to the description by Collins 

 and additional remarks by Howe and Hoyt, 1. c, p. 119, differs 

 from our plant by its, on the whole, smaller dimensions, by its 

 differing ramification, — the erect filament being much rami- 

 fied below, rarer above, — and by its smaller sporangia 

 6 X 15 ^. 



The Acrochætium spec, was found at a depth of about ten 



meters. 



St. Croix: off Frederikssted. 



21. Acrochætium bisporum Borgs. 



22. — occidentale Borgs. 



23. — comptum Borgs. 



24. — Avrainvilleæ Borgs. 



25. — hormorhizum Borgs. 



26. — Hypneæ Borgs. 



27. — repens Borgs. 



The host plant in which this species was found was Hypnea 

 muscijormis. 



Creeping with its basal part in the thick membrane of Grij- 

 fithsia globijera an Acrochætium was found which I prefer to 

 consider as a form of this species, until more material can be 

 examined. 



The plant (Fig. 420) has long, irregularly ramified, endophytic 

 filaments creeping throughout the thick membrane of the host 

 (Fig. 420 a). Now and then from these basal filaments erect ones 

 are given off. The cells of the basal filaments are subcylindrical 

 to oval being thickest in their middle, the filaments by this get- 

 ting a more or less moniliform appearance; the cells reach a breadth 

 of up to 11 /i and are two to three times as long. They have a 

 parietal chromatophore with a parietal pyrenoid. I have not 



