CRUSTACEA OF NEW ZEALAND. 219 



C. Iticifitgiis has both rami absent and the iieduncle itself reduced [56, pp. 143- 

 U6]. 



This varialjility of the third uropoda is only what \\c might have expected from the 

 affinity of the genus to Niphargus, where the third uropoda are also very variable, 

 diflfering iu length in the two sexes, and often being greatly elongated. Although 

 Crmujomjx evidently comes very close to Niphargus, it appears to be a good genus, and 

 to differ constantly from Niphargus in the more robust body and in the telson, which is 

 always entire and never cleft as in Nip)hargus. This diflfercnce in the telson is very 

 striking, and is somewhat remarkable, as it appears to give us some insight to the 

 direction that the development of the telson has taken in this group ; for if we are to 

 look upon Gammariis as representing one of the older types among the Amphipoda, as 

 suggested by Stebbing [108, p. xvi], from which Niphargus and Crangoniix have 

 successively developed, then it is evident that the development must have bci^n fi-om the 

 double telson of Gammarus to the deeply-cleft telson of Ni2)harg us, and then '.to the 

 single entire telson of Crangonyx. Considerations such as these naturally give rise to 

 the questions : — What is the use of the telson ? and why is it double in some species, 

 deeply cleft in others, and entire in others again ? But in propounding such questions 

 we only draw attention to our ignorance, and a much more complete knowledge of the 

 habits of these animals must be gained before we can hope to give any solution. In 

 Crangonyx mucronatus, Forbes, which Packard leaves in the genus Crangonyx, though 

 the species is, he says, perhaps entitled to rank as the type of a new genus, there appears 

 to be a great difference between the male and the female in the development of the 

 telson. Eorbes thus describes the two: — 



".The telson of the male is a smooth cylindrical appendage, usually abou.t as long as 

 the first three abdominal segments, and as large as the last joint of the pedicel of the 

 lower antenna. It presents a very slight double curve, is obliquely rounded at the end, 

 and tipped by a cluster of short hairs. In some cases this appendage is half as long as 



the body In the female this (the telson) is very similar to the telson of 



C. gracilis, Smith. It is flattened and slightly emarginate, a little longer than broad, 

 extending to the tips of the second pair of anal legs, and bears two terminal clusters of 

 spines of four or five each." Quoted from Packard [83, pp. 37, 38]. 



I am not aware of any other sjiecies of Niphargus or Crangonyx in which there is a 

 difference between the sexes in the telson, and a cylindrical telson half as long as the 

 animal is so remarkable that I was at first almost inclined to suspect some mistake ; but 

 specimens lately received from Mr. W. P. Hay, of Irvington, Indiana, agree in all 

 respects with Mr. Forbes's description ; but, as he points out, there can be little doubt 

 that the species should form the type of a separate genus distinct from Crangonyx. 



The species that I have to describe, Crangonyx compactus, is remarkable in that the 

 three pairs of pleopoda have each only one branch instead of two, as in almost all Amphi- 

 poda, the inner branch being apparently the one that is absent. These examples are 

 sufficient to show how imperfect our knowledge of the Amphipoda still is, and what 

 startling variations may be found when least expected. 



It would be interesting to know whether the other species of Crangonyx agree with 



SECOND SERIES. — ZOOLOGY, VOL. VI. 29 



