76 



In general outline and in the rounded lateral corners to the 

 last thoracic segment in both sexes, the form approaches near to 

 that of A. tonsa Dana and to A. giesbrechii (Dahl), while in 

 size (i.oimm to 1.12mm for the mature female; 92mm to i.oomm 

 for the male) it approximates closely to measurements given for 

 both the above forms. In the presence of rostral filaments (rather 

 in the shape of attenuated hooks than filaments) it also approaches 

 the two mentioned species. 



In A. tonsa, the anterior antennae of the female reach not quite 

 to the posterior margin of the genital segment, while in mature 

 forms of A. gracilis these organs reach well over the margin, 

 but in immature specimens only to the end of the last thoracic seg- 

 ment. 



As in A. tonsa, the abdomen is relatively short, being a little 

 less than one-fourth the length of the thorax, while there are no 

 thorns on the genital segment. The female of A. gracilis differs 

 from that of A. tonsa in the absence of hairs to the abdominal 

 segments; the only hairs present on the numerous specimens ex- 

 amined from all localities mentioned being confined to the inner 

 and outer margins of the furca, although the anal segment of the 

 male is rather densely haired on the margins, while the fine spines 

 present on the second abdominal segment of the male of A. tonsa 

 are absent in A. gracilis 



In A. gracilis, the genital segment of the female is equal to 

 the two following; the 2d and 3d about equal, and the furca 

 equal to the 3d, with the proportion of length to width as 8:6. 

 In the male, these characters approach very near to A. tonsa. 



The fifth feet of the female are practically the same as figured 

 for A. tonsa, having the curious barbed-like projections to the 

 middle of the attenuated apical claw, while the lateral setae are 

 plumose as in the latter species. 



From 7 to 8 very minute points are present at the rounded 

 corners of the last thoracic segment of the male. 



A. giesbrechii differs from A. tonsa mainly in having only 

 the anal segment very sparsely haired and in the somewhat 

 greater length of the anterior antennae. In these respects, A. gra- 

 cilis approaches nearer A. giesbrechii than A. tonsa, and these 

 diflferences are of so minor character as to lead to the belief that 

 A. gracilis and A. giesbrechii are one and the same species, but 

 without a comparison with Dahl's types the question could hardly 

 be settled. If they be identical, Dahl's specific name will have to 

 give way to that of Herrick, the latter having published A. gra- 

 cilis seven years previous to the description of A. giesbrechii, 



