238 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



GENUS LYNCEUS. 



Sub-Genus Alona — Section A, Alona vera. Section B, AloneVa. 



Subgenus Pleuroxus — Section A, Pleuroxus verus. Section B, Leptorhynchus.^ 



Characters of Percantha and Rhypophilus are combined in the species 

 P. procurvus^ Birge, so that one must be dropped or new diagnoses 

 formulated. I am not sure that the same species is not at first Pleu- 

 roxus verm"- and only later assumes the form known as Rh)/po2)hiliis. 

 So with Pereantha the amount of serrature of the posterior margin is 

 in part a question of age. 



SUB-GENUS ALONA. 



This group contains two sections which resemble each other in form 

 and, in general, in detail; but it is exceedingly difficult to formulate 

 a diagnosis that shall strictly limit it. The form is generally sub- 

 quadrangular with rounded corners; the terminal claw is armed with 

 but a single spine at the base; the beak is rather short; and the pre- 

 vailing marking consists of longitudinal lines. 



Section A, Alona (vera) Balrd. 



This genus contains a large number of minute animals which are 

 widely distributed. 



The authors who have done the most to elucidate this genus are 

 Schoedler, P. E. Mueller and Kurz. Birge has contributed most 

 largely, thus far, to the knowledge of American species, which are, for 

 the most part, identical or very close to the European. No other 

 genus is so difficult among the Lynceidce, for the most minute differ- 

 ences are relied upon to distinguish species. The species of this genus 

 are not greatly altered by the production of the winter eggs. The 

 males are frequently but little smaller than the opposite sex, and are 

 recognized by the altered form of the post-abdomen and the presence 

 of a hook on the first foot. The form is more perfectly rectangular 

 than in the next section; the shell is only exceptionally reticulated 

 and very rarely tuberculate, occasionally smooth. The lower angle 

 of the shell is not armed with spines, but is generally rounded. There 

 is only one basal spine upon the claw of the post-abdomen, which 

 usually bears a row of scales beside the anal spines. The antennae 

 have eight setse. The claw of the male post-abdomen is removed from 

 the lower angle. 



Many species are known, all of which that seemed recognizably de- 

 fined have been included in the following key, which is believed to 



1 Instead of Harporhynchus, a name preoccupied in zoology. 



2 Embryos of P. procurvus have tlie part which is to be curved forward attenuated before leaving 

 the brood cavity, however. 



