278 Records of the Indian Museum. [VoL. V, 
Owing to the great differences which exist between young ana 
old individuals, and between the two sexes, considerable difficulty 
is often experienced in determining the species of Palaemon. Asa 
rule, new species should not be described in the absence of a series 
of individuals of both sexes at different periods of growth, and 
certainly never from individuals which do not appear to be adult. 
In some cases where adult males show clearly marked differences, 
e.g., P. rudis and P. idae, very young individuals of the same 
species are almost indistinguishable, and the adult females can 
only be separated with considerable difficulty. Descriptions of 
new species which have not been largely based on an examination 
of adult males are therefore likely to be misleading, and lack of 
attention to this general principle has largely contributed to the 
numerous synonyms with which the genus Palaemon is overbur- 
dened. 
The most reliable specific characters are those derived from 
the form and relative length of the joints in the larger chelipedes 
of adult males, and the general form and toothing of the rostrum 
in both sexes. In opposition to Ortmann we do not attach much 
importance to the shape of the telson-tip, which sometimes varies 
greatly in members of the same species ; but we find that useful 
characters can frequently be obtained from the relative lengths of 
the two sub-terminal spinules when compared with the telson-tip. 
The division of the genus into four groups, Eupalaemon, Brachy- 
carpus, Parapalaemon and Macrobrachium, which has been proposed 
by Ortmann, is of doubtful utility, for the characters on which 
they are founded depend to some extent on the age of the indivi- 
dual. Thus P. scabriculus, when young is a Eupalaemon, when 
older a Parapalaemon, and very old males might be placed in the 
group Macrobrachium. 
In the case of our species the examination of a large number 
of individuals of different sizes seems to establish the following 
facts in regard to the modifications of structure which accompany 
growth :— 
1. The rostrum in the voung is relatively longer than in the 
adult. 
2. ‘The larger chelipedes, always shorter than the body in > 
the young, are usually much longer than the body in 
the adult. In species with the chelipedes of the second 
pair very unequal, the latter part of this statement 
only applies to the larger chelipede. 
The merus and carpus of the larger chelipedes as a rule 
remain of the same relative length. 
4. The ischium grows much more slowly than the merus and 
carpus, with the result that while in the young it is 
either equal to or longer than the merus, in the adult 
it is always shorter than the merus. 
5. The palm grows much faster than the merus, carpus and 
fingers, so that while usually shorter than the carpus 
Oo 
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