Saat 
“$ 
SCOTTISH FRESHWATER PLANKTON. 543 
Genus Stavrastrum, Meyen. 
15. 8S. curvatum, West, in Journ. Linn. Soc., Bot. xxix. 
(1892) p. 172, t. 22. f. 13. 
The specimens were almost exactly like the original Irish 
examples. 
Long. sine spin. 31-32°5 1; lat. sine spin. 31-35 4; lat. cum 
spin. 71-75 pw; long. spin. 21-23 p; lat. isthm, 65-8. (Pl. 17. 
fig. 12.) 
Hab. Loch Shin, Sutherland. 
16. S. sacuLIrERUM, West, 1. c. t. 22. f. 14. 
There seems little doubt that Arthrodesmus longicornis, Roy, 
is merely a biradiate form of S. jaculiferum in which the spines 
are not quite so divergent. Bédrgesen (‘ Freshw. Alg. Ferées,”’ 
in Botany of the Ferdes, part i., Copenhagen, 1901, p. 282, t. 8. 
f.1) has observed specimens from the Faroe Is. in which one 
semicell was biradiate and identical with that of Arthrodesmus 
longicornis, the other semicell being triradiate and agreeing with 
that of typical Staurastrum jaculiferum. Borge (in Botan. 
Notiser, 1897, p. 213, t. 3. ff. 5, 6) has also found the biradiate 
form in plankton material collected in the Isle of Mull. 
We find considerable variability in the amount of divergence 
of the spines, and the three forms of S. jaculiferum generally 
met with are :— 
Forma srrapiata. (<Arthrodesmus longicornis, Roy.) <A 
variable form, sometimes with a longer isthmus. (Pl. 17. fig. 1.) 
Long. sine spin. 21-27 p; lat. sine spin. 17-20»; long. spin. 
25-31 «3; lat. isthm. 5°5-7°5 w; crass. 12°5 p. 
Hab. Loch a Gharbh Bhaid Mhoir, Sutherland. Loch Mor 
Bharabhais, Lewis ; and Loch Skealtur, N. Uist, Outer Hebrides. 
Loch Tay and Loch Katrine, Perthshire. Loch Doon, Ayrshire. 
Forma rrrrapiata. (Pl. 17. figs. 2, 3.) 
Long. sine spin. 21-24 y; lat. sine spin. 17-18 w; long. spin. 
24-35 uw; lat. isthm. 7°5 p. 
Hab. Loch Mor Bharabhais, Lewis, Outer Hebrides. Loch 
Doon, Ayrshire. Loch Tay, Perthshire. 
Forma quaprirapiaTa. (PI. 17. fig. 4.) 
Long. sine spin. 23; lat. sine spin. 17-19; long. spin. 
29-31 »; lat. isthm. 6 p. 
Hab. Loch Mor Bharabhais, Lewis, Outer Hebrides. 
