52 
Transactions of the Boyal Society of So2tth Africa. 
the middle layer of the zygospore-membrane. This seems to be due to the 
fact that the lamellae of the middle layer are undulate, these undulations 
appearing as wavy lines, and, when viewed exactly from above, as dots or 
little papillae (cf. Fig. 22, G and H). It is possible that this feature of the 
zygospore is not really distinctive of the variety, as earlier observers may 
not have had mature zygospores of 8. fiuviatilis before them ; immature 
zygospores, in the present material, showed no trace of the reticulation. 
Altogether our knowledge of 8. fiuviatilis is so meagre that it has been 
difficult to arrive at an exact conclusion as to the status of the present form. 
The cell-walls, both longitudinal and transverse, are strongly thickened 
and often lamellated (Fig. 22, 0). The conjugation-tubes are broad, thick- 
walled and very short (Fig. 22, F), as appears to be characteristic of 8. 
fiuviatilis {cf. Borge's figure, loc. cit.). Many of the thi^eads showed well- 
marked attaching cells (Fig. 22, A), generally simple (Fig. 22, A and B),. 
but occasionally branched (Fig. 22, E). The cells immediately following on 
these rhizoids were either of the ordinary type, or several of them were 
exceedingly thick-walled and provided with very attenuated contents in 
which no details of structure could be distinguished (Fig. 22, A and B) ; 
now and again similar cells were observed in the course of the filaments. 
This variety somewhat approaches 8. fallax (Hansg.), Wille {8. insignis 
(Hass.), Kuetz., yslw fallax, Hansgirg, Hedwigia, 1888), in which the middle 
layer of the membrane of the elliptical zygospores is brown with reticulate 
thickenings ; but this species frequently possesses folded end- walls, the 
chloroplasts run almost straight or exhibit fewer turns, and it does not 
appear to have special attaching cells in the mature condition. 
6. 8pirogyra rivularis (Hass.), Eabenhorst, Fl. Europ. Alg., iii, 1868,. 
p. 243. (Syn. : Zygnema rivularis, Hassall, Brit. Freshw. Alg., 1845, p. 144, 
PI. XXYII, figs. 1, 2.) (Fig. nostr. 23.) 
Samples 70 (common) and 71. 
Lat. cell, veg., 37-40 /x; long., 96-180^; crass, membr., 4*5-6 /i. I 
provisionally refer this form, of which only sterile material was present, to 
the above species, with which it seems to agree in dimensions, in the presence 
of two to three chloroplasts making several turns in the cells (Fig. 23, A), 
and in its habitat. The chloroplasts were very broad, with large pyrenoids, 
and were generally so densely arranged that it was difficult to distinguish 
the one from the other (Fig. 23, A). The membrane was usually very 
strongly thickened and stratified (Fig. 23, A and B), as also were the septa 
(which in extreme cases attained a thickness of 30 fi) . The lower end of 
the filaments terminated in a characteristic strongly thickened attaching cell 
(Fig. 23, B and C), with a broadened base and scanty chlorophyll-content;, 
in some few cases the cell next above was also provided with rather scanty 
contents. The upper ends of the filaments were neatly rounded off. 
