Birge—Notes on Cladocera. 
1051 
plainly does not promise any improvement. The other tend¬ 
ency is best represented by Sars: that of multiplying small 
genera for those species which seem to be only doubtfully 
assignable to the older and larger genera. 
Whatever may be done in future we are, I think, compelled 
to recognize the following facts at present: 1 
1. The species of Chydorina are readily distinguishable. 
They seldom intergrade and are not very variable. The genera 
are in many cases ill-defined and in some cases seem to be 
indefinable. 
2. In the center of the family is a large assemblage of 
species, clearly enough distinguished specifically; but which 
it is impossible to place in well-marked and easily separable 
genera. There are certain centers around which many of 
these species may be grouped. Such are Alona, Pleuroxus, 
CJvydorus; the last two hardly separable. But when these 
groups of species have been taken from the assemblage there 
remains a number of species not assignable to any of these 
genera, though obviously related to one or more of them; re¬ 
lated also to each other, though without many definable char¬ 
acters in common. These species may, for the sake of con¬ 
venience, be included in the genus Alonella. 
3. A second and smaller group of species can be distin¬ 
guished, more readily divisible into genera; perhaps because 
fewer species are known. This is the Alonopsis group, contain¬ 
ing that genus with Euryalona, Pseudalona, Acroperus, and 
as its extreme type, Camptocercus. These, as the names of 
some indicate, are related to Alona in the larger group. 
4. Several genera, each with one or a very few species, are 
also related to Alona but are developed in different directions 
from that taken by the preceding group. Such are Graptole- 
beris, Leydigiopsis, Leydigia, and Leptorhynchus. 
5. Several genera, each also with one or few species, are 
marked by the development of a number of characteristics to 
such a degree that they are far removed from the central 
group. These are Anchistropus, which seems related to Chy- 
dorus; Dwihevedia, Monospilus and Dadaya, which seem to 
look toward Alona. 
