18 U.S. NATIONAL MUSEUM BULLETIN 259 



except that coxal plate of 4 is larger than that of 3. Pereopod 7 

 usually as long as or often a little longer than pereopod 6, which is 

 in turn longer than pereopod 5. Brood plates of females relatively 

 small, extending ventrally as far as segment 4 of pereopods 2, 3, and 4 

 but shorter on pereopod 5. Two or three median sternal gills or 

 processes present or absent on pereonites 2, 3, and 4; single pair 

 of simple sternal gills or processes present or absent on 1st pleonite; 

 paired, lateral sternal gills present on pereonites 6 and 7, simple or 

 bifurcate; paired, biarticulate coxal gills present on pereopods 2-6, 

 present or absent on 7. 



Abdominal side plates: posterior margins convex or nearly straight, 

 with 1 or more stiff setae each; posterior corners small and blunt, or 

 sometimes subacute (but rarely acute), or sometimes indistinct or 

 absent; small spines present on ventral margins of 2 and 3 but often 

 absent on 1. Pleopods decreasing in overall length posteriorly, outer 

 ramus a little longer than inner ramus; rami of pleopod 1 a little 

 longer than rami of 2 and distinctly longer than rami of 3; peduncles 

 with 2 coupling hooks each on inner distal margin, peduncle of pleopod 

 2 usually subequal to 3 in size but often a little longer and broader 

 than 1. Uronites 1-3 fused, although a rudimentary or occasionally 

 a distinct suture is often visible between 1 and 2. Uropod 1 of male 

 with distal peduncular process extending up to 1/4 the distance along 

 inner facial margin of outer ramus with possible exception of only 

 one species. Uropod 3 short, uniramous; single ramus shorter than 

 peduncle, armed with one or several apical spines. Telson as long 

 as or often longer than broad, armed apically and sometimes laterally 

 with spines; apical margin sub truncate, convex or sometimes with a 

 shallow excavation. 



Affinities. — Until the systematics of all genera of the Crangonyx 

 section have been thoroughly revised, it will be difficult to assess 

 completely the relative values of their similarities and differences. 

 On the basis of available data, however, a number of significant 

 morphological parallels between Stygonectes and other North American 

 crangonycid genera can be illustrated as shown in table 2. When 

 major generic characters are compared, it is easily seen that Stygonectes 

 is morphologically more closely alUed with Apocrangonyx, Bactrurus, 

 and Stygobromus than with either Crangonyx or Synurella. It is of 

 further significance to note that all of the first four genera exclusively 

 inhabit subterranean waters and completely lack eyes and pigment. 

 Synurella, on the other hand, is unknown from subterranean waters 

 in North America, and although it is represented to some extent in 

 ground-water habitats of eastern Europe, the majority of its species 

 possess both eyes and pigment. Crangonyx has invaded subterranean 

 waters throughout its extensive range, but presently there appear to 



