420 



BULLETIN 15 8, UNITED STATES NATIONAL MUSEUM 



Female. — Carapace orbicular, as wide as long, the median lobe 

 squarelj^ truncated, or slightly concave. Fourth segment nearly as 

 wide as genital segment, its dorsal plates small, orbicular, and sep- 

 arated by a wide posterior sinus. 

 Genital segment half the width of 

 the carapace, longer than wide, 

 with nearly parallel sides and a 

 concave posterior margin; no leg 

 rudiments visible. Abdomen 2-seg- 

 mented, segments equal in length, 

 the basal one the wider. Exopods 

 of first three pairs of legs with 

 small horny claws; fourth legs 

 1-segmented, not visible dorsally; 

 two corrugated chitin ridges in 

 place of a furca. Total length, 

 10-12 mm. 



Male. — Carapace longer than 

 wide, widest posteriorly; fourth 

 segment long and narrow, less 

 than half the width of the genital 

 segment and without dorsal plates. 

 Genital segment narrow, spindle- 

 shaped, and squarely truncated 

 posteriorly, with small spinelike 

 lobes at the corners; two pairs of 

 leg rudiments present. Abdomen 

 2-segmented, segments equal in length, the terminal one slightly 

 wider. Total length, 7-7.75 mm. 



Remarks. — The absence of posterior processes on the genital seg- 

 ment is the best means of identification for this species. It is fairly 

 common around Woods Hole but has not been found anywhere out- 

 side of this area. 



ALEBION GRACILIS Wilson 

 Figure 264 

 Aletion gracilis Wilson, Proc. U. S. Nat. Mus., vol. 31, p. 704, pi. 18, 1907. 



Occurrence. — Found on the outside surface of the common dogfish, 

 the spiny dogfish, the sand shark, the dusky shark, the pollack, and 

 the bonito. 



Distribution. — Outside the present area a single lot of specimens 

 has been sent to the United States National Museum from the Laysan 

 Islands in the Pacific Ocean. 



Color. — Body a transparent cartilage gray, without pigment 

 markings. 



Figure 263. — Alehion glaber: a. Female, 

 dorsal (drawn by A. H. Baldwin, Sep- 

 tember 21, 1887) ; b, female, fourth leg 



