STATE GEOLOGIST. 91 



VI. — Genus Crepidocercus, Birge. 



The characters of this group place it rather near Alonella or be- 

 tween that and Pleuroxus. Form sub-quadrate with rounded 

 angles; dorsal line uniformly arched, terminating in a sharp angle 

 behind; lower margin convex, armed behind with a single spine as 

 in Pleuroxus unidens, and along the entire length with loose setae. 

 Beak of moderate length, acute. Post-abdomen deeply incised in 

 the anal region; lower posterior margin straight, rounded at the 

 apex; ventral margin straight or concave; claws with a single basal 

 spine and a few teeth. The post-abdomen is shoe-shaped and 

 armed with transverse rows of sette. 



The antennae are large, having eight setae and the usual spines. 

 Shell smooth or reticulate. 



Sp. 1 . Crepidocercus setiger, Birge. 



(Plate F. Fig. 13.) 



Length 0.4 mm. to 0.5 mm. Minnesota specimens measured 5 

 mm. This, the only species of the genus, is but rarely encounter- 

 ed, and is so peculiar as to be easily recognized when seen. Alona 

 intermedia has a post-abdomen with clusters of bristles, but in 

 Crepidocercus the post-abdomen is more as in species of Graptoleberis 

 than any other genus. The markings upon the shell are very 

 indistinct. 



Yii. — Genus Lynceus, 0. F. Mueller. 



The perplexing inter-relations between the three genera Alona» 

 Alonella and Pleuroxus give rise to the utmost confusion. No two 

 authors are agreed as to their respective limits, and the points given 

 by Kurz, who has carefully gone over the ground, are obviously 

 insufficient. Although there may be practical benefits to be derived 

 from the continuance of the nomenclature in use for groups which 

 in the general view can be distinguished, the value from a theoreti- 

 cal standpoint is reduced to a minimum. 



The genus Camptocercus (including here Acroperus, which differs 

 solely in the form of the abdomen, as a sub-genus) passes through 

 Alonopsis into the group represented by Alona. Leydigia, although 

 very near such forms as Alona quadrangulata, may be conveniently 

 distinguished as a transition to species like Ilyocryptus. 



