312 Birge — Notes on Cladocera. 



There is no constant difference between the species in size r 

 color or transparency. Hellich says that the striae are not so 

 closely set in angustatus as in leucocephahis. 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. " 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. 



Monospiltts tenttiro«tris, Fischer. 



This species occurred in dredgings from Twin lakes, Toma- 

 hawk lake, and lake Julia, Wisconsin. I have also received it 

 in collections from Easthamptoif, 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 Lynceida3. In Alona and allied genera, as Acroperus and 

 Camptocercus, the suture marking the junction of the shell of 



