60 U.S. NATIONAL MUSEUM BULLETIN 2 59 



spine and 4 small spines on outside, 5 small spines on inside; posterior 

 margin relatively short, with 3 sets of setae; lateral setae in transverse 

 rows, singly inserted. Gnathopod 2: propod palmar margin less 

 concave, armed with a double row of 5 or 6 spine teeth; posterior 

 angle with 1 large spine and 5 smaller spines on outside, 6 small 

 spines on inside; posterior margin with 6 sets of setae; lateral setae 

 hi transverse rows, mostly singly inserted; coxal plate rather broadly 

 roimded anteroventrally, with 6 or 7 marginal setae. Coxal plate of 

 pereopod 3 a little broader than long, with about 8 marginal setae; 

 that of pereopod 4 about as broad as long and with 7 to 10 marginal 

 setae. Pereopod 7 between 45 and 50 percent as long as body, a 

 little longer than pereopod 6 and 40 to 45 percent longer than pereopod 

 5. Bases of pereopods 5-7: posterior margins generally straight; disto- 

 posterior lobes well developed, rather broad. Pereopod dactyls rela- 

 tively short. All gill structures present. 



Abdominal side plates: posterior margins convex, with 2 or 3 setae 

 each; posterior corners poorly developed, bluntly rounded; ventral 

 margins of plates, 1, 2, and 3 with about 2, 4, and 4 spines, respectively, 

 Pleopod 1, inner ramus nearly twice as long as peduncle. Uropod 1 : 

 outer ramus nearly as long as inner ramus, only about 60 percent as 

 long as peduncle, armed with about 8 spines; inner ramus with 7 to 9 

 spines; peduncle with up to 16 spines. Uropod 2: outer ramus 75 

 to 80 percent as long as inner ramus, about 50 percent as long as 

 peduncle, armed with 5 spines; inner ramus with 10 spines; peduncle 

 with about 4 spines. Uropod 3, ramus up to and usually more than 

 50 percent as long as peduncle, armed with 4 or 5 apical spmes. 

 Telson nearly twice as long as broad; armed apically with 11 or 12 

 unequal spines, the outer ones on either side usually deflected laterally 

 from the midlme and possessing 2 or 3 small tmes on inner distal 

 margin. 



Female. — Differing from the male as follows: Antenna 1 with 19 

 or 20 primary flagellar segments. Antenna 2 only about 2/3 the length 

 of antenna 1 in larger specimens, with 9 or 10 flagellar segments, the 

 penultimate segment being normal. Gnathopod 1, palmar margin of 

 propod less concave. Propod of gnathopod 2 generally more setose 

 but otherwise similar. Pereopods 5-7 not as slender in sexually ma- 

 tiu-e females as in sexually mature males. Uropod 1, rami with 19 or 

 20 spines. Telson with untined, apical spines. 



Variation. — Sexual dimorphism in S. tenuis s. lat. is marked by 

 the enlarged second antenna of the male. The singular diagnostic 

 value of this phenomonal structure as well as its probable ethological 

 significance in sex recognition is biologically notable. In further re- 

 gard to this structure, it should be pointed out that S. tenuis is the 



