92 TWELFTH ANNUAL REPORT. 



Phrixura, Graptoleberis aud Crepidocercus, each containing few 

 species v/-hicli can be readily recognized, fill a place in the system; 

 but it is practically impossible to distinguish Alonafrom Pleuroxus 

 without instituting the very indefinite genus Alonella to contain a 

 variety of small intermediate forms. Percantha, Rhypophilus, 

 Harporhynchus and Pleuroxus seem to be pretty generally regarded 

 as constituting a single group which may be recognized by the 

 long rostrum, high shell and greater development of the antenna 

 bristles. Alona, on the other hand, with its broader fornices, shorter 

 beak, fairly developed antennae, and more rectangular shell, is, per- 

 haps, the pivotal point of the group. According to this view, then, 

 the old name Lynceus is revived for the aggregate; and the other 

 names are retained, in part, as titles of largely'' conventional groups 

 or sub-genera, thus: 



Genus Lynceus. 



Sub-genus Alona. 



Section A. Alona vera. 



Section B. Alonella. 

 Sub-genus 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 Pleuroxus verus ^ and only later assumes th»? form known 

 as Rhypophilus. So with Percantha 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 prevailing marking consists of longitudinal lines. 



Section A. Alona {vera). Baird. 



This genus contains a large number of minute animals which 

 are widely distributed. 



1 Instead of Harporhynchus, a name preoccupied in zoology. 



2 Embryos of P. procurvus have the part which is to be curved forward attenuated 

 before leaving the brood-cavity, however. 



