No. 407.1] THE CREST OF DAPHNIA HYALINA. 881 
with a low even crest and terminal claw deprived of basal 
denticles, and D. pellucida with a large crest and a terminal 
claw ornamented at its base by a series of slight denticles." 
D. gracilis Hellich was distinguished from the forego- 
ing mainly by its great height of evenly rounded crest, and 
was believed to be closely allied to D. galeata.  Eylmann (86) 
was the first to recognize the close relationship among the 
forms hitherto described. He classed them all as varieties 
of D. hyalina. Matile (90) takes the same view. I translate 
from Eylmann : 
“In the course of my work the conviction was more and 
more forced upon me that all Daphnia possess a tendency 
toward the formation of local varieties, and indeed these 
variations are not limited to one part of the body, but extend 
to all parts; now it is the shape of the head, now the form of 
the valves, and again the size and development of the append- 
ages, which give rise to the distinction as a variety. However, 
the differences which arise in one habitat are very slight, 
and extend to only a few individuals; but if we compare 
Daphniz from separate localities, the differences are so strik- 
ing that a doubt often arises as to whether the specimens are 
of the same or of different species, and often the question 
cannot be decided until transition forms are found in other 
localities which make possible a gradual change from one form 
to another.” He cites D. hyalina as the best example of this 
character, and gives as a reason for these manifold deviations 
from the type the fact that the species is rigidly confined to 
lakes, the consequent isolation offering very favorable condi- 
tions for the origination of new forms. Richard (96), in his 
* Revision des Cladoceres," included the above-mentioned 
forms, together with D. plitvicensis Sostaric! and D. rectifrons 
Stingelin,? in the single species D. Aya/ina, with the remark : 
* D. hyalina est d'ailleurs trés variable, comme on le verra plus 
loin." 
The American Representatives of the Species. — These have 
been but little studied. A brief description of two varieties 
1 Low, rounded crest. 
2 Similar to var. galeata. 
