7i 
Tetraedroides spetsbergensis 
clades, and the many chloroplasts and pyrenoids and the extrusion 
of spores from a lateral pore also point somewhat to that group. If 
so, Tetraedroides might be considered as a much reduced member of 
the Siphonoclades, possibly derived from a Rhizoclonmm-like ancestor, 
and bearing a relationship to the group similar to that of Pleurococcus 
to the Ulotrichales. These surmises are, however, far too speculative 
to use as a definite basis of classification. 
It might be suggested that this alga, whatever its systematic 
position may be, is a filamentous form which has assumed the bi- 
cellular thick-walled habit as a protection against the rigour of the 
climatic conditions prevailing in the situation in which it lives. The 
incipient thread formation and the relatively thin-walled thalli might 
be considered as summer stages, and the thick-walled polyhedral forms 
perhaps as winter stages/ 
DIAGNOSIS. 
Tetraedroides spetsbergensis gen. et sp. nov. 
Thallus piriformis ellipsoidalis tetrahedralis aut polyhedralis, ex 
cellulis duabus (sed rarius tribus vel quatuor) compositus. Muri 
exteriores crassi, interiores tenues. Cellula uninucleata; chloro- 
plastides multae quarum aliquae unam pyrenoidem habent. Multi- 
plicatio vegetationalis per filum incipiens quod in geminas cellulas 
constrictione et muris transversalibus divisum est. Multiplicatio 
asexualis per sporus quatuor et nonmotiles qui ex cellula per foramen 
laterale exeunt. Thallus long. 25-42 /jl; lat. 10-25 /jl; crass, max. muri 
4 /x. Habitatio inter muscos in rimis saxorum in Insula Spetsbergensi. 
Department of Botany, 
Armstrong College, University of Durham. 
January, 1923. 
Explanation of Plate II 
Camera-lucida drawings, x 650 
Figs. 1-7. More common type of thallus. In Fig. 4, the nuclei and chloroplasts 
are shown; in Fig. 7, ditto, some of the chloroplasts contain pyrenoids. 
Figs. 8-11. Less common polyhedral thalli; Fig. 10 shows axial view. 
Fig. 12. Three-celled thallus; empty specimen. 
Fig. 13. Four-celled thallus; nuclei shown. 
Figs. 14-19. Incipient thread formation. In Figs. 14, 15 and 16 the cell- 
contents of the tubular portions are somewhat plasmolysed. In Fig. 17 c, 
the upper part of the thread is segmenting by constriction but the trans¬ 
verse walls have not yet appeared. In the lower part, the two lowest cells, 
b, are apparently aborting, and cell a is completely aborted. 
Fig. 18. Partial constriction and formation of thick transverse splitting wall 
at a. At b, thin interior transverse wall is about to form. 
Fig. 19. Group of pairs of cells formed by complete constrictions alternating 
with thin transverse-wall formation. 
Fig. 20. Formation of four spores (?), of which three are already extruded from 
the cell via a lateral pore. 
