SECTION III 

 KEY TO GENERA OF NORTH AMERICAN FRESHWATER ASELLIDAE 



Lateral margin of head produced to form thin plate covering or over- 

 hanging base of mandible, this plate frequently but not always 

 incised (Fig. 8A) . Anterior margin of head with pointed median 

 protuberance (carina) between bases of antennules (Fig. 8A) . 

 Distal segment of exopod of pleopod 3 (outer gill operculum) 

 approximately hemispherical in shape, with division between it 

 and proximal segment running obliquely backwards from inner 

 distal angle (Fig. 9A) : Liraeus Rafinesque 



(species not identified here; see Introduction) 



Lateral margin of head not produced to cover or overhang base of man- 

 dible (Fig. 8B) . Anterior margin of head without a median protu- 

 berance between bases of antennules (Fig. SB). Distal segment of 

 exopod of pleopod 3 (outer gill operculum) sub-quadrangular in 

 shape, division between it and proximal segment running approxi- 

 mately at right-angles to long axis of specimen and commencing 

 well anterior to inner distal angle of appendage (Fig. 93): 



Asellus St. Hilaire (page 19) 



Fig. 8. Dorsal view of 

 head: A, Liraeus sp.; 

 B, Asellus sp. Original 

 ca.=carina, e.=eye. 



Fig. 9. Third pleopod: A, Liraeus 

 sp.; B, Asellus sp. Original. 



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