134 Illinois State Laboratory of Natural History. 



inner distal angle with a short thick recurved hook with 

 smooth edges, extending hut little heyond the joint. This is 

 merely the continuation of the hyaline lamina at the side of 

 the segment. 



Second basal segment of the right fifth leg of the male 

 (PI. XXIII., Fig. 1-5), seen from behind, irregularly trape- 

 zoidal in form, very broad distally, and about twice as long 

 as its narrowest part is wide. On the outer distal angle of 

 this segment is another projection, equal to or greater in 

 length than the inner ramus. This also shows great varia- 

 tion, and is either rounded or acute or even acuminate at 

 the apex. First and second segments of outer ramus sub- 

 quadrate, the second about as wide as the first and about two 

 and a half times as long. About a third the length of the 

 second segment from its base is a considerable contraction, 

 the width here being about half the width of the broadest part. 

 Slightly below the middle, on the outer margin, is a spine, 

 minutely serrate at the inner edge. This is generally long 

 and straight, about half the length of the segment, but varies, 

 and is sometimes shorter, thicker, curved, and less than one 

 third the length of the segment (PL XXIV., Fig. 4 ; PI. XXY., 

 Fig. 3-5). Terminal hook rather long and slender, slightly 

 and sometimes sinuously curved, about one and a fourth 

 times the length of the preceding segment. The inner margin 

 is serrate, beginning about the middle of the hook and con- 

 tinuing to the tip. 



Inner ramus of the right fifth foot wanting, a peculiarity 

 rarely found among the American species of Diaptomus, but 

 approached most closely by I), lintoni Forbes and D. minutus 

 Lilljeborg, in which the ramus is very small, almost rudi- 

 mentary. The ramus is represented by an immovable spine, 

 minutely spinose at the tip. This is greatly diverse in shape 

 and sometimes gives indications of a joint (PI. XXIII., Fig. 

 2), as if a case of ankylosis. 



Left fifth foot of the male biramose; second basal segment 

 quadrate, with a short thick spine just above the outer distal 

 angle. Second segment of outer ramus irregularly subquad- 

 rate, about two thirds as wide as long, provided at the inner 



