242 DR. C. CHILTON ON THE SUBTEREANEAN 



rounded, and eacli bearing a fine hair ; the hinder margin slightly concave. Fig. 17 

 is taken from a female ; in one male specimen examined the hinder margin was slightly 

 more concave. 



Sexual differences. The female differs from the male as above described in many charac- 

 teristic features, and especially in the gnathopoda, so that if they had not been taken 

 together it is rather unlikely that the two forms would both have been assigned to the 

 same species. With the facts before me, however, I have no doubt that we have to deal 

 with the two sexes of the one species. The one form I consider the male from its largely 

 developed gnathopoda, from the absence of brood-plates &c., although I have not actually 

 found fully- developed spermatozoa in it. The other form is undoubtedly a female, for I 

 have frequently taken it bearing eggs; it is found associated with the male both at 

 Eyreton and Ashburton ; it is much smaller, the largest specimens being not more than 

 two-thirds the size of the largest male, and it is very much more numerous. Although I 

 have not yet obtained a complete series of transitional forms, still from the facts given 

 below I feel convinced that in this species, as in many other Amphipoda, the young male 

 at first resembles the female in the form of the gnathopoda &c., and that the peculiar 

 characteristics of the male are not acquired until the animal has attained to sexual 

 maturity. 



In the female the antennce are more slender than in the male ; in accordance with 

 the smaller size of the animal, the setse on them are less prominent, especially in the 

 lower antenna?, and the calceoli are wanting. I have found calceoli in some specimens 

 which in other characters closely resembled females, but, as I have already said, these are 

 more probably immature males. As the calceoli in these specimens are of very small 

 size, extremely delicate and transparent, and hence easily overlooked, I thought at 

 first that they were perhaps present also in the females, though I had failed to see 

 them ; however, I have since looked for them very carefully in several mature females 

 and can find no trace of them. 



In the upj)er lip, the mandibles, and the lower lip the female does not appear to differ 

 from the male. 



In the first maxilla the inner lobe is larger and bears fully twice as many plumose setse 

 as in the male specimen drawn in tig. 10, PI. XXII. It is quite possible that the number 

 of plumose setse on the inner plate of this maxilla bears some relation to the size of the 

 animal irrespective of its sex ; but all the female specimens examined have about twice 

 the number found in the male, and I have not had sufficient male specimens of different 

 sizes to be able to decide how many of these setae are usually present, or whether 

 the one figured is exceptional or not. 



In full-grown females the setae on the middle lobe and at the end of the palp closely 

 resemble those of the male as drawn, the two palps being different as in the male. 



In the second maxilla in tlie female the oblique row of plumose setse contains about 

 twice as many setse as in the male figured in fig. VI, n, and the remarks made in con- 

 nection with the first maxilla apply to the second maxilla also. 



In the maxilliiiedes (fig. 11, PI. XXIII.) we find a very considerable difference between 

 the two sexes (compare fig. 15, PI. XXII., and fig. 11, PI. XXIII.) . In the female the two 



