﻿172 PROCEEDINGS OF THE NATIONAL MUSEUM vol. lOO 



length of right leg from the point of origin to the distal tip of the 

 hook or claw as long as the abdomen exclusive of caudal setae. 



Discussion on classification. — This species is placed in Kief er's genus 

 Arctodiapto-rnus because (1) the endopodite of female fifth leg termi- 

 nates in hairs only, (2) tip of right endopodite of male fifth leg is 

 acuminate, without seta, only hairs, and (3) the distal end of left 

 exopodite is produced into a pincerlike structure, though compara- 

 tively small. The fact that the male right first antenna has no spine 

 on the fourteenth joint precludes it from Kiefer's subgenus Arcto- 

 diaptomus s. str., and the unmodified distal half of the second joint of 

 the male fifth exopodite on the right side excludes it from the sub- 

 genus Stenodia'ptonms^ which has the distal half of this joint greatly 

 reduced in diameter. This leaves the subgenera Rhadodiaptomus and 

 Haplodiaptomus for comparison. The present species resembles 

 Haplodiaptonnus in the possession, in both sexes, of distinct fourth and 

 fifth thoracic segments, instead of having them partially fused, and in 

 the absence of a thorny process on the antepenultimate joint of the 

 male right antenna, except a thin hyaline membrane on the side of its 

 distal part. But this form differs from Haplodiaptomus in many im- 

 portant points : (1) The endopodite of the right fifth leg of the male is 

 not cylindrical throughout its length, but acuminate and with fine 

 hairs all around this tip instead of on the truncate end only; (2) 

 there is a slight indentation on the mesial side of this ramus, almost 

 suggesting the condition in E odlaptomnus ; (3) the spine on the thir- 

 teenth joint of the right first antenna of the male instead of being ap- 

 parently small is very large and conspicuous, being many times the 

 size of the other spines on the antenna; (4) the fifteenth joint, follow- 

 ing the others, also has a spine. Although this species resembles 

 Rhdbdodiaptomus in the possession of a pincerlike structure on the 

 distal end of the endopodite of the fifth left leg in the male and in lack- 

 ing a spine on the fourteenth joint of the male right first antenna, it 

 differs from this subgenus in ( 1 ) that the terminal pincerlike structure 

 is much smaller, (2) the fourth and fifth thoracic segments are not 

 fused in either sex, (3) the antepenultimate joint of the male right 

 first antenna is without a spine on its distal end, and (4) the second 

 joint of the right exopodite of the fifth leg in the male has no hyaline 

 appendage. 



From both of these two subgenera erected by Kiefer the Arctodiap- 

 tomus from Erh Hai can be distinguished by the excessive length of the 

 spine on the thirteenth joint of the male right first antenna, the ab- 

 sence of any spiny or other form of process on the second exopodite of 

 the male fifth right leg besides the lateral spine and the terminal claw, 

 and the presence of a hyaline elevation on the mesial aspect of the 

 proximal exopodite of the same ramus instead of one on the second 



