312 
Birge—Notes on Cladocera. 
There is no constant difference between the species in size,, 
color or transparency. Hellich says that the striae are not so 
closely set in cmgustatus as in leucocephalus. In my specimens, 
the reverse is true although the difference is not great. The 
figures of P. E. Mueller (P. Ill, f. 15, 18.) show the facts as I 
find them. Mueller (’67, p. 169.) says of A. angustatus “margo 
ventralis * * * * postrema parte glabra. 5 ’ Hellich’s figure 
of angustatus shows it ciliate to the tip. My specimens show 
a similar arrangement. 
The antennary setae in my specimens of A. leucocephalus do> 
not quite reach the posterior edge of the valves when the an¬ 
tennae are reflexed. In many specimens of A. angustatus they 
are as long as in many individuals of the other species. The 
crest of the'head is very variable in both sexes. The post-ab¬ 
domen affords no constant characters for distinguishing the 
species. 
On the~whole there seem to be two fairly well marked forms- 
of the genus Acroperus ; but general shape and appearance are 
the only means for discriminating them, as the characters re 
lied upon by different authors for specific marks are not con¬ 
stant. The straight dorsal margin, the short antennae, and 
the position of the posteal teeth, are characteristic of A. 
angustatus , but these structural features show considerable vari¬ 
ation in the direction of the 'corresponding structures of A , 
leucocephalus. 
Monospilus tenuirostris, Fischer. 
This species occurred in dredgings from Twin lakes, Toma¬ 
hawk lake, and lake Julia, Wisconsin. I have also received it 
in collections from Easthampton, Mass., and Isle Royale, Mich. 
Herrick reports it from Minnesota. 
In all my bottles cast shells were far more plenty than ani¬ 
mals. The cast shell shows clearly that the affinities of the 
genus are with Alona. It is well known that the relation 
of the shell of the head to that of the valves varies greatly in 
the Lynceidce. In Alona and allied genera, as Acroperus and 
Camptocercus , the suture marking the junction of the shell of 
