Smith—Monograph of Scenedesmus. 
477 
inermibus, extimis utroque apice saepius dorso armatis.” 
Dimens. cell. 11x3.5, 12.5x4.5, 15x6, 16x6, 16.5 x 6 g. 
Long. acul. 10-12 g. 
This species is the most widely distributed of any of the 
genus. Calling anything with four horns S. quadricauda 
and failing to observe the accessory horns that separate this 
species from S. longus Meyen and S. abundans (Kirch.) 
Chod., probably accounts for many reports of it, although in 
spite of these possible errors of identification the species un¬ 
doubtedly occurs in abundance everywhere. 
Chodat (1913) has added considerably to the confusion 
in the systematic classification of the species Scenedesmus 
by his work in pure cultures. He describes many new species 
but since the bases for their specific differentiation is physio¬ 
logical rather than morphological, it is difficult to identify his 
species, especially since he gives no cell measurements. There 
is no justification for the establishing his S. quadrispina , 
S. longispina , and S. nanus as distinct species; since they 
are all so clearly related to S. quadricauda , in having spines 
only on the poles of the terminal cells. His work is of value 
to the systematist, however, in pointing out that it is possi¬ 
ble to isolate strains which vary within certain spacial lim¬ 
its, while others with the same external morphological char¬ 
acters vary within other spacial limits. This is particularly 
true of the alga known as S. quadricauda. 
Reinsch (1867) attempted to classify these varieties on 
the basis of their size but failed to name the different groups 
into which he separated them. Since his work was not done 
on material in pure culture, the limits he set for variations 
were largely a matter of judgment rather than accurate 
knowledge. 
The varieties described here have all been obtained in 
pure culture; so that the exact limits of their variation is 
accurately determined. In spite of the fact that a consid¬ 
erable number of varieties of S. quadricauda are described 
I believe that field material can be identified by means of 
the cell measurements given. 
The form for which the name S. quadricauda is retained 
has been isolated several times from various stations in this 
vicinity. It is selected as the typical form because its di¬ 
mensions are about in the middle of those given by de Toni 
for the variations in size for the cells of S. quadricauda. 
