SUBTERRANEAN AMPHIPOD STYGONECTES 21 



EMARGINATUS GrOUp 



Diagnosis. — Almost exclusively cavernicolous. Adult size range 

 up to 18.00 mm. Little or no pronounced sexual dimorphism except 

 that where both sexes are known, females are larger than males. 

 Antenna 1 usually from 40 to 70 percent as long as body, typically 

 about twice the length of antenna 2; accessory flagellum short, seldom 

 much exceeding 1st segment of primary flagellum in length. Mouth 

 parts corresponding approximately to those figured for S. emarginatus 

 but varying specifically where noted. Mandible well developed, 3rd 

 segment of palp with up to 12 long setae. Maxilla 1, inner plate with 

 up to 9 long, plumose setae. Maxilla 2, inner plate with up to 10 

 long, obliquely placed, plumose setae. Maxilliped: outer plate ex- 

 tending nearly to or just beyond apex of palpal segment 1; inner 

 plate with 4 to 5 thick spines apically. Inner lobes of lower Hp 

 typically small or poorly developed. 



Propod of gnathopod 1 about equal in size to propod of gnathopod 

 2 but broader proximally; palmar margin obhque; posterior angle 

 with 2 to 6 large, unequal, often slightly curved spines on outside; 

 posterior margin short, without setae except in one species (S. 

 stelbnacki, new species). Gnathopod 2: posterior angle of propod 

 with a single large spine on outside (excepting S. stellmacki, which 

 also has 2 smaller spines); posterior margin of segment 5 usually 

 without rastellate setae. Coxal plates of pereopods 2 and 3 up to 

 30 percent longer than broad, rather narrowly rounded distally. 

 Coxal plate of pereopod 4 about as broad as long, extending distally 

 for about 40 to 45 percent the length of segment 2. Pereopod 7 

 equal to pereopod 6 in length or sometimes slightly longer, approxi- 

 mately 20 to 30 percent longer than pereopod 5. Abdominal side 

 plates: posterior margins tj^ically convex but occasionally nearly 

 straight, usually with relatively few setae; posterior corners commonly 

 well developed and sometimes acute. Single ramus of 3rd uropod 

 relatively short, with 1 to 5 apical spines. 



Relationship.— A rather highly specialized and diversified group 

 which, with few exceptions, is easily distinguished from all other 

 species groups of Stygonectes by the loss of setae on the posterior 

 margin of the first gnathopodal propod; absence of small spines on 

 the outside of the posterior angle of the second gnathopodal propod ; 

 and by the well developed and sometimes acute posterior corners of the 

 abdominal side plates. Morphologically, the emarginatus group is 

 most closely allied to the flagellatus group, of central Texas, with 

 which it appears to share a number of significant characters. 



Ecology and distribution. — The emarginatus group ranges 

 from central Pennsylvania south through central and western Mary- 



