1910.| J. R. HENDERSON & G. Marruar: Freshwater Prawns. 279 
in the young, it equals or sometimes even exceeds the 
latter joint in the adult. 
6. The fingers (except in the case of P. :dae) grow a little 
more quickly than the carpus and merus, but less 
rapidly than the palm. 
The following sexual characters, though common to other 
Macrura, are of some practical importance ; the thoracic legs of 
the female are not so approximate at their bases as in the male, 
and this is particularly the case with regard to the last pair of 
legs; the ventral surface of the abdomen is also wider in the 
female, and the abdominal epimera which form a lateral protection 
for the eggs attached to the abdominal appendages, are of greater 
depth, especially those of the first three segments. Of much greater 
importance, however, are the special sexual characters of the 
genus. The female is smaller than the male. The rostrum in 
the female is usually comparatively longer than in the male, 
and in species in which it is upturned distally that of the female 
is more distinctly curved. The chelipedes in the female are always 
much thinner and shorter than those of the male. The spinosity 
and pubescence of the chelipedes, carapace and legs, which frequent- 
ly characterise males, are much less strongly developed and in 
some cases even totally absent from females. The fingers of the 
female are of equal length, and the teeth or tubercles on their 
ridges weak or absent, while their tips are less curved than those 
of the male. Coutiére has drawn attention to an apparent prepon- 
derance of males in the genus Palaemon, but we imagine that this 
is partly due to a process of selection by collectors, for in most of 
our species females were met with in as great numbers as males. 
The subject of dimorphism in the males of Palaemon is one of 
considerable interest. While the young individuals of any species 
are all very much alike, it is not till a later stage, when the maxi- 
mum size of the individual is practically attained, that the male 
characters definitely assert themselves. In many if not in all 
species, two forms of male are to be met with, vzz., normal males 
usually of considerable size, with the larger chelipedes specially 
developed, and males of the second type (‘‘males féminisés” of 
Coutiére) generally smaller but sometimes attaining the same size 
as normal males, in which the chelipedes resemble those of females. 
Dimorphism of the males has been recorded in several species 
of Decapod Crustacea, and it has been shown, notably by Faxon 
in crayfishes of the genus Cambarus, that the two forms represent 
breeding and non-breeding stages, which alternate in the life 
history of each individual. In order to ascertain if any similar 
alternation exists in Palaemon, we have recently (September and 
October) examined the reproductive organs of a large number of 
males of P. malcolmsonii, P. dubius, and P. scabriculus, but with 
negative results. We find that in these three species many males 
of the second type, with the larger chelipedes undeveloped, appear 
to be sexually mature ; their testes are well-developed, and in all 
