554 MR, W. WEST AND PROF. G. S. WEST ON 
Staurastrum Ophiura, S. Arctiscon, and WS. grande in the 
plankton, as all three species are extremely rare in the bogs of 
the western areas in which they occur. 
6. As a striking contrast to the abundance of Desmids, there 
is a most remarkable scarcity of many of the free-swimming 
Protococcoidee, which the researches of Lemmermann, Chodat, 
Schréder, and others have shown to be relatively abundant in 
the lakes of Switzerland and Germany. Amongst those Proto- 
coccoidee noted only the genus Spherocystis is exclusively 
confined to the plankton, although many genera which are com- 
monly more abundant in tie plankton than elsewhere, such as 
Kirchneriella, Botryococcus, Dictyospherium, &c., were present 
in quantity. 
7. The most striking and characteristic Diatoms are Asterio- 
nella gracillima, Tabellaria fenestrata var. asterionelloides, and 
forms of Swrirella robusta. Two other interesting species are 
fhizosolenta eriensis and FR. longiseta. 
8. The majority of the species of Staurastrum and Arthro- 
desmus which occur in the plankton are remarkable for their 
long spines or long processes with spinate apices. Even those 
species which are normally long-spined increase the lenzth of 
their spines when in the plankton. The genus Xanthidium, all 
the species of which are spined, is also one of the commouest 
genera of the plankton. Again, in the Protococcoidee, the 
subfamily Phytheliez is almost an exclusively plankton group, all 
the species of which are remarkable for their armature of long 
spines. 
This excessive development of spines, which occurs amongst 
the plankton Diatoms as well as amongst the Chlorophycee, is 
obviously for protection, and has been rendered necessary owing 
to the assumption of a purely free-floating existence. 
9. The plankton is much richer in species in the late summer 
and autumn than in the spring. 
10. Several very interesting new species have been obtained 
from the Scottish plankton. Staurastrum conspicuum is one of 
the largest and most striking British species of the genus. 
Genicularia elegans is the second known species of the rarest of 
all the genera of Desmids. Arthrodesmus quiriferus is remark- 
able for the length of its spines, and the triquetral variety of 
Micrasterias Murrayi merits special mention on account of the 
additional evidence it affords towards a correct idea of the 
evolution of the genera of Desmids. 
