424 Wisconsin Academy of Sciences, Arts, and Letters . 
(1883). He was also the first to describe species based on 
the external sculpturing of the cell wall, a method which has 
been used almost entirely ever since for establishing new 
species. The work of de Toni (1889B) is by far the most 
complete that we have for the distribution of the various 
species and is a critical compilation of the species that were 
known at that time but is quite inadequate at present, in 
1893 de Wildemann also published a monograph of the 
genus, but his material was drawn to a large extent from de 
Toni’s data. The chief point in the work of de Wildemann 
is the uniting of S. quadricauda and S. bijuga into one spe¬ 
cies called S. variabilis but even this possesses no originality 
since Franze had previously united these two species under 
the emended name S. obtusus (Meyen) Franze. 
At the time this work was started there was no modern 
collection of the species of the genus but during its prepara¬ 
tion two notable publications have appeared. In 1913 
Brunnthaler presented a key to the species of Scenedesmus 
with figures of the species he recognized. This has been of 
great service in checking up data that I have secured. In 
the same year Chodat published an extensive monograph 
based on pure culture studies, the greater portion of the work 
being carried out on the genus Scenedesmus. It is a matter 
of regret that I must take so critical an attitude towards his 
work, but it is practically impossible to determine the dif¬ 
ferent species he describes. Few cell measurements are 
given and the only differences between many of the species 
are physiological. Chodat himself admits the impossibility 
of the worker in the field identifying many of his species. 
The cultural work that I have carried on shows that the 
shape of the cell, the presence of four terminal horns, the 
presence of horns in addition to those at the four corners of 
the coenobe, lateral ridges, and teeth are characters that are 
constant in any one culture isolated by pure culture methods. 
The relative arrangement of the cells is a character that is 
constant in certain species and variable in others. In the 
present work the presence of any one of these characters 
which persists in a pure culture is taken as the character 
that distinguishes a single species. The combination of two 
of these characters in a single coenobe is one that is gener¬ 
ally regarded as also constituting specific rank and this prac- 
