North American Species of Diaptomus. 171 



as long as the segment is wide ; inserted near the beginning 

 of the distal third ; slightly curved and distinctly denticulate 

 on the inner margin. Terminal hook rather slender, a little 

 more than twice as long as the preceding segment ; not regu- 

 larly curved, but divided approximately into thirds by abrupt 

 angles, the upper one very sharp, below which on the inner 

 margin, the hook is delicately but distinctly denticulate. 



Inner ramus of right fifth foot short, one-segmented, reach- 

 ing just to, or extending very slightly beyond, the end of the 

 first segment of the outer ramus ; apex bluntly triangular and 

 delicately hairy. 



First segment of the outer ramus of the fifth leg of the 

 female subquadrate, about twice as long as wide ; second seg- 

 ment slightly shorter than the first, moderately curved, the 

 inner margin distinctly denticulate ; third segment wanting, 

 being represented by two spines ; the outer short and thick 

 and only about half as long as the inner. 



Inner ramus of fifth leg of female, one-segmented, extending 

 slightly beyond the end of the first segment of the outer ramus ; 

 delicately hairy at the apex and on the distal fourth, and 

 armed in addition to this with two slender spines about as long 

 as the shorter of the two representing the third segment of 

 the outer ramus. 



Length of female, 1.1395 mm.; of male, 1.0248 mm. 



The above measurements are those of Professor Marsh. 

 The largest female I examined was 1.13 mm. in length, the 

 smallest, 1 mm. ; while the largest male I measured was 1 

 mm. in length, the smallest, .96 mm. 



Prof. Marsh originally described this species, and I am 

 greatly indebted to him for the specimens from which the 

 above description was made. He found D. reighardi in only 

 three localities, all in Michigan ; North Lake, on Beaver 

 Island, Intermediate Lake, and Crooked Lake. I do not 

 know of its having been recorded from any other place. 



At first sight D. reighardi, in respect to the fifth pair of 

 legs of the male, is very like D. oregonensis Lilljeborg, but 

 the details of structure are very different in the two, and there 

 can be no doubt as to the validity of the species. 



