STATE GEOLOGIST. 107 



abdomeu is slender, usually truncate and armed behind with a single 

 set of sharp teeth on either side; the terminal claw has usually two 

 spines and may be serrate. 



The male has a shorter beak, the post-abdomen is more or less 

 modified, and the first foot has a powerful hook. The winter eggs 

 frequently have a true ephippium; and sometimes this structure is 

 like that of Chydorus, toward which the round forms of this genus 

 seem to lead. There are upwards of a dozen valid species, several of 

 which are American. 



Key id Section A, Pleuroxus verus. • 



§ Beak not curved forward. 



A. Shell reticulate. 



(a) Post-abdomen very narrow. 



1. P. hastatus, Sars, 



2. P. stramineus, Birge. 



(b) Post-abdomen not very slender. 



* Terminal claws with two spines. 



3. P. trigonellu-', O. F. Mueller. (?) 

 ?. P. ornatus, Schcedler. 



** Terminal claws with a single spine. 



4. P. acutirostris, Birge. 



B. Shell smooth, except upon the front margin. 



5. P. adunctus, Jurine. 



C. Shell striped, 



(a) Shell very long and low. 



* With one tooth below. 



10. P. unidens, Birge. 



** Without a tooth ; female with a hook upon the first foot. 



8. P. hamatus, Birge. 



*** Without a tooth on the shell or claw on the foot. 



9. P. afflinis, Herrick. 



(b) Shell high. 



* Lower angle spined. 



t Autennce with eight setae, anterior margin of valves toothed. 



7. P. denticulatus, Birge. 

 tt Antenupe with seven setae. 



6. P. hairdii, Schoedler. 



** Whole posterior margin of shell spined (Percantha.) 



11. P. truncata, O. F. Mueller. 



§§ Beak procurved (Rhypophilus.) 



A. Shell reticulate. 



* Faintly and regularly. 



13. P. glaher, Schoedler. 

 '** Strongly and irregularly. 



