SUBTERRANEAN AMPHIPOD STYGONECTES 49 



extended distoanterior lobe of 7th pereopod basis in sexually mature 

 males; and by the long, laterally spined telson. Largest males, 

 18.75 mm; largest females, 15.75 mm. 



Corresponding closely to the descriptions by Shoemaker (1938, 

 1942a) with the following additions and modifications: 



Male.— Antenna 1 only 15 percent longer than antenna 2 in very 

 large specimens. Gnathopod 1: palmar margin of propod generally 

 straight, obUque, distally with a small, armed notch or excavation and 

 armed throughout with a double row of 10 or 11 spine teeth; posterior 

 angle with 2 large spines and 3 or 4 small spines on outside, about 

 5 small spines on inside; posterior margin relatively short and with 

 4 or 5 sets of setae ; lateral setae in transverse rows, doubly and singly 

 inserted. Gnathopod 2: propod palmar margin relatively short, 

 straight, armed with a double row of 6 to 9 spine teeth; posterior 

 angle with 1 large spine and about 6 small spines on outside, 7 or 8 

 small spines on inside; posterior margin long, with 7 to 10 sets of setae; 

 lateral setae in transverse rows, more numerous than on gnathopod 1 , 

 in sets of 2's, 3's, and sometimes 4's; coxal plate with up to 11 or 12 

 setae marginally. Coxa] plate of pereopod 3 with up to 12 marginal 

 setae; that of pereopod 4 with up to 15 marginal setae. Segment 4 of 

 pereopod 5 broadly expanded, about 1/2 times as wide as corresponding 

 basis. Gill arrangement complete. Structure of uropods 1 and 2 about 

 as shown in figure 10. Uropod 3: peduncle with 1 or 2 spines; ramus 

 with 3 unequal, apical spines. Telson about twice as long as broad, 

 armed laterally with several sets of spines and apically with about 10 

 spines. 



Female. — Gnathopod 1: propod palmar margin shghtly convex, 

 oblique, armed with a double row of about 12 spine teeth; posterior 

 angle with 2 large spines and 4 or 5 smaller spines on outside, 4 to 6 

 small spines on inside; posterior margin with 4 to 6 sets of setae. 

 Gnathopod 2: propod palmar margin generally straight, relatively 

 short, armed with a double row of 7 or 8 spine teeth; posterior angle 

 with 1 large spine and 5 to 7 smaller spines on outside, 6 to 8 small 

 spines on inside; posterior margin with about 8 sets of setae. Segment 

 4 of pereopod 5 not broadly expanded, only about 1/3 as wide as 

 corresponding basis. Gill arrangement complete. Telson armed with 

 several sets of lateral spines and 10 to 13 unequal, apical spines. 



Variation. — Sexual dimorphism in this species is pronounced as 

 exemplified by the male secondary sex characters. A significant 

 difference between the size mean of sexually mature individuals in a 

 number of different populations has been noted as shown by the data 

 in table 4. Data given for males may not be as conclusive as that 

 given for females because of the greater difficulty in determining the 

 size at which sexual maturity is attained. The most reliable character 



