BIOLOGY OF THE SCOTTISH LOCHS 
303 
first inspecting a drawing of it, suspected its affinity to Jl. subkasti- 
ferum^ an opinion since justified by the finding of many examples 
having one semicell of the type and the other of the variety. It 
appears like an incipient species ; yet, considering its abundance 
in Loch Morar, and the facilities for dispersal into other lochs im- 
mediately adjoining, its restriction to this loch is remarkable. The 
continual occurrence of semicells of the type and variety in the 
same example shows how little fixity the form has. All this sug- 
gests the query whether the physical properties of the water of a 
loch may not give rise directly to peculiarities of species. The 
absence of the family Daphnidcie from Loch Morar is another un- 
explained fact in distribution. 
Palmellaceae. — Tetraedron limnetmmi, Borge. — Confined to the 
extreme north of the mainland ; only known from Loch an Ruathair 
in Sutherland, and Lochs Shurrery and More in Caithness : a Scandi- 
navian species. 
Diatoms. — Asterionella graclUima^ Heib., and A. formosa^ Hass. — 
Although Asterionella is one of the commonest plankton organisms, 
the two species are rarely found together. Messrs West doubt 
whether the two species, which difier chiefiy in the relative slenderness 
of the cells, are really distinct, and the fact that they are not to any 
extent mixed in the lochs gives support to this theory. Though both 
are widely distributed, A. graciUima appears to be more a Highland 
and western form, A. formosa more Lowland. In some small Lowland 
lochs A. formosa is greatly reduced in size, and the colonies are 
cruciform, having only four cells. 
Fragilmia crotonensis^ A. M. Edw., var. contorta^ West. — The 
type of the species is common in the north and west, but the 
strikingly beautiful variety is of much more restricted range. It 
is only known in Sutherland, where it was first obtained in some 
small lochs on the west coast. Afterwards it was found in three lochs 
of the Helmsdale basin, in E. Sutherland, Lochs nan Cuinne, Baddan- 
loch, and an Ruathair. It was from plankton from Loch an Ruathair 
sent to Mr West that the variety was described. 
The filaments are short, apparently of a definite and uniform 
length, and are very strongly twisted, so that each looks like a pair 
of fans joined together. 
Tahellaria fenestrata (Lyngb.) and T. flocculosa (Roth.). — Both 
species are usually present together. T. fenestrata is very often in 
spiral colonies (var. aster'ionelloides). The stellate form of T, flocculosa 
is, on the other hand, very rare, and has only been seen in one or 
two lochs. 
