STAURASTRUM. 21 



Shiel, Inverness ! Loch Brandy (at 2080 ft.), Clova, 



Forfar ! Loch Shubhaill and in 5 other lochs in Lewis, 

 Outer Hebrides ! 



IRELAND. --In lakes, Clifden to Roundstone, Bally- 

 nahinch, and rare in the plankton, Galway ! Dublin 

 and Wicklow (Arch.). Lough Fea, Londonderry ! 



Geogr. Distribution. Switzerland. Norway. Sweden. 

 Finland. Faeroes. Iceland. India. United States. 

 N.W. Canada. Patagonia. 



This species bears a superficial resemblance to St. mucronatum, 

 but differs in its larger size and more angular semicells. It is 

 more abundant in plankton than in other situations. 



Var. scoticum W. & G. S. West. (PL CXXXI, figs. 



9, 10.) 



? Staurastntni meyacanthmn forma Borge in Algol. Xotiser 1897, p. 213, 



t. 3, f. 7. 

 St. megacanthuni var. scoticum West & G. S. West, Scott. Fresh\v. Plankton,. 



I, 1903, p. 544, t. 16, f. 8 ; Further Contrib. Plankt. Scott. Lochs, 1905, 



p. 485. 



Apical margin of semicell straight or slightly concave ; 

 sinus a little more widely open ; spines relatively longer, 

 and slightly diverging. 



Length, without spines, 35-44 a ; breadth, without 

 spines, 38-51 \i ; including spines 79-1 \\\L ; length of 

 spines 19-34fj, ; breadth of isthmus 10'5 \L. 



SCOTLAND.- -Plankton of Loch Shin, Loch nan Cuinne 

 and Loch Ghriama, Sutherland ! Mull (Borge). Loch 

 Doon, Ayr ! Loch Langabhat, Lewis, and Loch Laxa- 

 dale, Harris, Outer Hebrides ! 



Geogr. Distribution. Scandinavia. Canada. 



This variety is readily distinguished from the typical form of 

 St. megacanthum by its slightly concave apex and strong, slightly 

 diverging spines. [In the opinion of the writer this Desmid 

 bears a strong resemblance to St. curvatum West, and might well 

 be placed as a form of this species rather than with St. mega- 

 Cftnthum. In deference to Professor West, however, it has been 

 retained in the position originally assigned to it.] 



