704 PROCEEDINOS OF THE NATIONAL MUSEUM. vol. xxxi. 



5. Bod}' of genital segment much wider (1.5 to 3 times) than long, with si)ines on 

 the margins only 6 



5. Body of genital segment a little longer than wide, the entire dorsal surface and 

 margins covered with spines; dorsal plates small and close together, 



difficllis van Beneden, 1892 



6. Dorsal plates small and widely separated; genital segment often entirely 

 smooth gracilis Wilson, 1905, p. 704 



6. Dorsal plates of good size and close together, their bases fused; genital segment 

 and processes with a heavy fringe of spines carchari<v Kroyer, 1863 



ALEBION GRACILIS « Wilson. 

 Plate XVIII, figs. 35-48. 

 Alebion gracile 'WiLSO's, 1905, p. 128. 



Female. — Carapace elliptical, a little longer than wide, prolonged 

 anteriorl}^ on the mid line over the frontal plates into a blunt rostrum 

 which just reaches the anterior margin of the frontal plates. Poste- 

 rior sinuses broad, somewhat enlarged at their bases, and dividing the 

 carapace into nearly even thirds. 



Both median and lateral lobes squarely truncated. Grooves sepa- 

 rating the areas arranged like the letter H as in Caligus, but with the 

 sides widely separated, making the lateral areas nari-ow and the median 

 area ver}^ Ijroad. 



Free thoracic segment nearly as wide as the genital segment, half as 

 long as wide, and bearing upon its dorsal surface a pair of semilunar 

 plates. These are parallel to the median line and some little distance 

 from it, their convex sides outward and their posterior ends somewhat 

 enlarged and overlapping the genital segment. This latter is a little 

 more than half the width of the carapace, widest at the center, and 

 prolonged posteriori}^ on either side into a stout conical spine which 

 reaches beyond the anal laminae. The sides of this segment and its 

 posterior projections carr}^ in the fully developed adult a fringe of 

 short spines while at the tips of the projections are three or four 

 longer ones. In immature females these spines at the tips of the pro- 

 jections are the only ones present. 



Abdomen two- jointed, the first joint considerably larger than the 

 second, and extending backward on either side of the latter in a })lunt 

 conical projection similar to those on the genital segment. The ter- 

 minal segment is strongly constricted at its junction with the first and 

 is only about half the width of the latter. It bears at its posterior end 

 a pair of anal lamina^ nearly as long as the segment itself, slightly 

 enlarged at their tips and curved in toward each other. E:ich lamina 

 carries four long plumose seta?. 



First antennse two-jointed as in Caligus, but not as heavily armed 

 with spines; posterior pair with a large basal joint and a slender, sim- 

 ple terminal claw attached at one side. 



« For change in gender see \^. 707. 



