REPORT OjST THE ISOPODA. 129 



the exception of the fourth, which is subequal to it ; the second joint is extremely short, 

 the fingers of the hand are stout and not greatly elongated, their extremities cross. 



The ambulatory limbs appeared to me to fall into two distinct series as in Paratanais, 

 the anterior three pairs being more slender than the posterior ; also the posterior appen- 

 dages are rather more spiny than the anterior, as is stated by Sars to be the case in 

 Leptognathia generally. 



The abdominal appendages are quite normal in structure and are all fully developed. 



The uropoda are about twice the length of the terminal segment of the abdomen ; 

 they are biramose, both rami being biarticulate ; the outer branch is two-jointed but very 

 slender, not being longer than the basal joint of the limb, its extremity is furnished with 

 a single long hair ; the inner branch is quite three times as long as the outer, as well as 

 being considerably thicker, the proximal joint is rather stouter as well as longer than the 

 distal. 



Station 149h, off Cumberland Bay, Kerguelen, January 29, 1874; lat. 48° 45' S., long. 

 69° 14' E.; depth, 127 fathoms; volcanic mud. 



Paratanais, Dana. 



Paratanais, Dana, U.S. Expl. Expect., Crust., vol. ii. p. 779. 



This genus differs from Tanais in the following particulars : (1) in the antennules which 

 differ in the two sexes, being better developed in the males than in the females ; (2) in the 

 greater size of the three posterior thoracic appendages as compared with the three anterior ; 

 (3) in the full development of all the abdominal limbs ; (4) in the short biramose uropoda, 

 each ramus being two-jointed. 



In his Eevision of the Tanaids 1 Sars only allows two species, one of which (Paratanais 

 forcipatus, Spence Bate and Westwood) is a European form, whde Paratanais elongatus, 

 Dana, is a native of the Sooloo Islands. 



During the voyage of the Challenger a considerable number of small Tanaids which 

 appear to me to belong to this genus, were dredged at Kerguelen, in shallow water. 

 They agree with it in every particular except in the extremely marked sexual dimorphism ; 

 not merely are the antenna? different in the two sexes but the chelae are extraordinarily 

 divergent in structure. This great discrepancy makes me hesitate before including all 

 the individuals in the same species ; the reasons, however, which appear to me to necessitate 

 this step are, firstly, that the chelae do frequently exhibit sexual differences in this as 

 well as in other genera ; and, secondly, that all the individuals with peculiarly modified 

 chelee are, judging from the antennules, males, while those specimens with chelae of the 

 normal form are invariably females. I believe, therefore, that I am right in simply regard- 

 ing this species as exhibiting in a very marked degree a dimorphism in the form of the chelae. 



i Archivf. Math, og Nat, Bd. vii. (1882), p. 32. 

 (ZOOL. CHALL. EXP. — PART XLVIII. — 1886.) Ebb 17 



