102 TWELFTH ANNUAL REPORT. 



There are usually but seven setae on the antenuee, or the eighth 

 is a minute hair; on the ramus having the lateral setae one of the 

 terminal setae is frequently reduced. In many cases the whole 

 shell is marked by minute stride in addition to the proper markings, 

 but this is also found in some species of the true Pleuroxus. Kurz 

 gives, as a character of Alonella, the presence of but a single basal 

 spine to the claw of the post-abdomen; but P. E. Mueller figures 

 two spines on the claws of one of his species (A. exigua), and 

 Schoedler figures eight setse on the antenna of A. excisa. American 

 specimens of A. excisa and of A. pygmaea both certainly have a 

 very minute eighth seta. There remains, therefore, positively no 

 point which can be relied upon to distinguish these little lynceids 

 from Pleuroxus or Alona. Perhaps, however, these species, as a 

 group, may be recognized by what has already been said. Three 

 species are found in Minnesota. 



A. Rostrum long, bent backwards. 



1. A. rostrata, Koch. 



B. tiostrum short. 



I. Lower ijosterior angle toothed. 



(a) Shell more or less reticulate. 



* Reticulated areas minutely striate. 



2. A. pulcheUa, Herrick. 



3. A. excisa, Fischer. 



** Reticulated areas smooth. 

 t Head depressed. 



4. A. exigua, Lilljeborg. 

 tt Head horizontal. 



(?) 5. A. grisea, Fischer. 



(b) Shell marked by lines running diagonally upward and backward. 



6. A. pijgmcea, Sars. 



II. Lower posterior angle smooth, shell longitudinally striate. 



7. A. striata, Schoedler. 



Sp. 1. Alonella rostrata, Koch. 



Lynccus rostratus, koch, lilljebokg, schoedler. 

 Alonella rostrata, sars, kurz, 

 Alona rostrata, p. e. mueller. 



Body long, rapidly narrowed behind; dorsal line strongly arched 

 in front toward the depressed head; the lower margin straight, with 

 — 3 small teeth at the angle. The fornices are broad, but the beak 

 is sharp; the pigment fleck is but little smaller than the eye, to 

 which it is three times nearer than to the beak. The post-abdo- 

 men is long, very much as in A. excisa, but longer. Length 0.4 — 



