THE FRESHWATER PLANKTON OF THE SCOTTISH LOCHS. 5138 
the oreater quantity of the Desmids brought into these new conditions of pelagic life 
find them most unsuitable for their further existence, and rapidly die. Thus a specific 
selection has been gradually going on, certain species only having been able to maintain 
their existence in the plankton. In some cases these species have succeeded not only 
in maintaining their existence as plankton-organisms, but in prodigiously increasing in 
numbers, and in further developing those characters which have proved of most use to 
them under their new conditions of life. 
The Family Cidogoniaceze is represented in the plankton of almost all the lakes by 
more or less fragmentary filaments of various species of Gidogoniuwm. These are mostly 
of very small size, and they never appear to produce oospores. They are most 
probably replenished from the margins of the lochs. 
In the Protococcoidez the Scottish plankton is not very rich, and in this fact there 
is a marked contrast between it and the plankton of some of the areas of continental 
Hurope. Thereis a striking absence of Richterella, Golenkuua, Lagerhermia, and many 
other genera which the investigations of CHopat, LEMMERMANN, SCHRODER, SCHMIDLE, 
and others have shown to occur abundantly in the lakes of Germany and Switzerland. 
Pediastrum Boryanum is general in almost all the lochs, but is never abundant, and 
the same is true of Ankistrodesmus falcatus. Crucigena rectangulars and C. 
wregularis were each noticed from several lochs, but they were not general, and no 
other species of the genus was observed. Most of the green Algze belonging to the 
Protococcoideze have a decided preference for shallower and warmer water than is met 
with in the Scottish lochs, so that their comparative scarcity is not very remarkable. 
The four most abundant were Botryococcus Braunu, Ineffiguata neglecta, Spherocystis 
Schroetern, and Gilaocystis gugas. 
Diatoms are very abundant in the Scottish plankton, and they do not disappear 
from it in May and June, but are very numerous even after that date. This abundance 
of Diatoms is largely due to the presence in quantity of a few species, most of those 
observed being relatively scarce, and principally littoral species, which have found their 
way to the pelagic region. The true plankton Diatoms which thrive in the surface 
waters of the Scottish lakes are chiefly— 
Tabellaria fenestrata and var. asterionelloides. EHunotia pectinalis. 
66 flocculosa. | Navicula major. 
Pragilaria Crotonensis. Vanheurckia rhomboides, var. Saxonica. 
Asterionella formosa. Surtrella biseriata, 
i gracillima. a robusta and var. splendida, 
To these may be added Synedra Ulnu, Meloswra granulata, several species of 
Cyclotella, Amphipleura pellucida, Rhizosolenia longiseta, and R. eriensis, which occur 
widely distributed, but in smaller quantity. 
We have remarked the curious absence of Cymatopleura from the plankton of most 
of the Scottish lakes, as this genus is commonly present in the plankton of shallower 
lakes. 
