FRESH-WATER PLANKTON 
349 
slender and elongated, the fourth spine being as well developed as 
the third, and the second, third, and fourth spines were widely 
divergent. In this loch Cei'atmm hirimdinella i-eached the furthest 
limit of its development (Plate XV. fig. 6). 
On l^^th August 1903, in Loch Skebacleit, the fourth spine was 
ill developed, but the second and third spines were long and diverging. 
On 14th August 1903, in Loch nan Deaspoirt, the surface tempera- 
ture being 58" '0 Fahr., all the specimens seen were rather small, and 
the fourth spine was ill developed (Plate XV. fig. 9). 
On the same date, in Loch Dhomhnuill Bhig, were found compara- 
tively slender forms with well-developed fourth spine ; the surface 
of the organism was somewhat prickly (Plate XV. fig. 7). 
On 15th August 1903, in Loch Valtos, the Ceratium was much 
the same as in Loch Dhomhnuill Bhig. 
In the lochs of the Orkney and Shetland Islands Ceratium 
hirimdiJiella was very abundant. 
Orkney. — On 15th August 1903, in Loch Kirbister, with a 
surface temperature of 57°'0 Fahr., the individuals were small, the 
fourth spine was moderately developed, and the other spines were 
rather short. 
On 22nd August 1903, in Loch Harray, the surface temperature 
being 55° '0 Fahr., the fourth spine was always present, though small ; 
the other spines were reduced in length, sometimes considerably so, 
as in Plate XV. %. 18. 
On 27th August 1903, in Loch Hundland, the surface temperature 
being 62" Fahr., some specimens were like those of Lewis, the third 
and fourth spines being fairly well developed and diverging rather 
widely ; other specimens had no fourth or only a very small one, 
whilst all the other spines were fairly long (Plate XV. figs. 24 and 25). 
Shetland. — -Here the individuals were always small, the spines 
short, and the body surface coarsely reticulate. 
On 2nd July 1903, in Loch Tingwall, the surface temperature 
being 55 ''3 Fahr., the second spine was fairly long, but all the others 
were relatively short, the fourth being almost absent (Plate XV. 
fig. 20). 
On 4th July 1903, in Loch Spiggie, the surface temperature being 
56° "8 Fahr., the fourth spine was usually absent, and all the other 
spines were short ; in general shape the organism was stumpy, its 
central body being large (Plate XV. fig. 19). 
On 5th July 1903, in Loch Brow, the Ceratium was very like that 
of Loch Tingwall ; many specimens had a small fourth spine. 
On 14th July 1903, in Loch Collaster, with a surface temperature 
of 53° '0 Fahr., the fourth spine was small or absent, the second spine 
was long, but the third spine was short (Plate XV. fig. 22). 
