POLYMORPHISM AMONG THE AMPHIPODA. 561 
clusion that we have a case of dimorphism wherever we find two 
forms of the male and only one of the female, for the two forms may 
be only different stages in the life-history of the same individual. 
This mistake appears to have been made in connection with the fresh 
water crayfish of North America, belonging to the genus Cambarus. 
In a paper in the “American Journal of Science,” Vol. XXVIL., 
January, 1884, Mr. Walter Faxon shows that the two forms of the 
adult male found in the species of that genus “are alternating 
‘periods in the life of the individual, the ‘first form’ being assumed 
‘during the pairing season, the ‘second form’ during the intervals 
between the pairing seasons.” He adds that it is possible that the 
two forms of the male found by Fritz Miiller in the genera Tanais and 
Orchestia may be explained in the same way as the two forms of the 
male Cambarus. In the case of the Zanais, if not also in that of 
Orchestia, this explanation is I think hardly admissable in face of ' 
Fritz Miller's distinct statement that the young males resemble the 
females until the last moult before sexual maturity, and that at that 
moult they change some into one form, some into the other, and that 
thereafter they appear to live only for love, having lost the moveable 
appendages of the mouth and being always found with the intestine 
empty. In other respects, too, the cases of dimorphism adduced by 
Fritz Miller seem to me to be hardly comparable with that of 
Cambarus. 
I have now to draw attention to two instances in the New 
Zealand Amphipoda in which the evidence, though not so complete 
as could be wished for, distinctly points to the fact that we have two 
species with more than one form of the male in each. in Mera sub- 
carinata the male differs considerably from the female in the form of 
the second pair of gnathopoda, and in the specimens from Stewart 
Island, described by Mr. Thomson under the name Mera Petrie, the 
whole lower surface of the propodos is very densely fringed with two 
rows of long simple hairs. I have collected specimens in Sydney 
Harbour which agree exactly with the description thus given by Mr. 
‘Thomson ; the hairs are very noticeable, for they are long and flexible 
and of the same size throughout their whole length and are thus 
readily distinguished from the ordinary stiff tapering sete some of 
which are also present on the same joint of the gnathopoda. In 
specimens from Lyttelton Harbour, however, these peculiar hairs are 
entirely absent, the propodos bearing only some of the usual kind of 
set, and both the propodos and the dactylos are slightly different in 
form; while in the females, of which I have specimens both from 
Lyttelton and Sydney, I can detect no difference whatever. It is 
quite possible that in this case the two forms of male may be simply 
two stages in the life of the same individual, as in the Cambarus, 
though I hardly think this can be the case, for of each form I have 
Specimens of various sizes; it is, again, worthy of notice that the two 
forms though taken with indistinguishable females, have not, as yet, 
both been taken in the same locality. 
The other instance that I have to refer to is from the genus 
Microdeuteropus, and here we have a species in which there are three 
different forms of the male to one of the female. In 1879, Mr. G. M. 
Thomson described a new species of Microdeuteropus (naming it WV. 
maculatus) from a female specimen taken in Dunedin Harbour. At 
