Chilton. — The New Zealand Species of Elasmopus. 



323 



but, as in the carination of the pleon and in all other characters they are so 

 closely similar, I consider them as belonging to the same species (Elasmopus 

 suhcarinatus). It is possible, as I pointed out in 1885, that the two forms 

 are alternating forms of the male, the setose one assumed during the pairing 

 season and the other during the periods between the pairing seasons— 

 in this respect resembling the alternating forms described by Faxon in some 

 species of Cambarus. I suggest the setose form as the one assumed during 

 the pairing season, because similar long slender setae are found as a dis- 

 tinctive male character in many Amphipoda, Isopoda, and other Crustacea, 

 and it is possible that they are sensory and of special use during the pairing 

 season. This explanation would perhaps also account for the fact that all 

 the males collected at any one time appear to belong either to one form or 

 the other, and that the two are not taken together, though, of course, this 

 and the fact that the non-setose form has not yet been recorded from 

 Australia would be more easily accounted for on the supposition that we have 

 two distinct species to deal with. However, Mrs. Sexton's results, which 

 prove that in Jassa fahata (Montagu) two forms of male occur, changing at 

 certain moults, induce me to think that we have only one species here also. 

 This species has been very fully described by Stebbing in his report on 

 the '* Challenger " Amphipoda, and the following brief description, based 

 upon his shorter diagnosis in " Das Tierreich,*' Gammwidea, will be suffi- 

 cient here. 



Spec ific D lag nos is . 



Female with, fourth pleon segment bicarinate behind the dorsal depression, 

 each carina ending in an acute tooth : third pleon segment with postero- 

 lateral corner acutely produced. First antennae elongate, sometimes as 

 long as the body, first joint about as long as the second but stouter and 



Fig. 1. — -Elasmopus suhcarinatus, male. Terminal portion of pleon 

 with uropoda and telson. 



bearing stout setae on the lower margin, third joint half as long as the 

 second, flagellum longer than the peduncle, markedly setose, accessory 

 flagellum well developed with six joints or less. Second antennae not much 

 longer than the peduncle of the first, ultimate joint of peduncle shorter 

 than the penultimate, flagellum shorter than peduncle. 



11 



