72 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
that a somewhat serious error has been made in the 
figure, the end of the ‘‘thumb”’ being shown 
emarginate, as in Jassa pusilla (Sars), whereas it is more 
or less pointed. The tooth on the dactylus is always 
present, though more prominent in some specimens than 
others; otherwise it nearly resembles Nebeski’s Im3, m4, 
in which, as in this form, the ‘‘thumb”’ springs from 
the middle and not from the base of the hind margin. 
That it is not the form precedent to the final moult is 
shown by fig. D and by an adult male specimen in the 
‘breakwater ’’ gathering, which has the right gnathopod 
normal and the left as in the young male. It is 
probably a dimorphic male form. 
The importance of these observations from the point 
of view of classification is obvious. As we have seen, 
ovigerous females in one gathering differ not only in 
size but also in the structure of the second gnathopods, 
so that the two forms, if taken in different localities, 
might easily be considered distinct species. So far as I 
know, this is the first record of dimorphism in females 
among the Amphipoda, though it is not uncommon in 
males. 
EXPLANATION OF PLATE. 
Jassa falcata (Montagu), Second gnathopods of : — 
Young, length 2 to 5 mm. 
Small ovigerous female, 6 to 7 mm. 
Large ovigerous female, 7:5 to 9 mm. 
Male during final moult, 10 mm. 
Male in the stage before D, 9 mm. 
Form described as P. herdmani, A.O.W., 6mm. 
Adult males (the ‘‘thumb’”’ in the small one 
is broken), 7 to 10 mm. 
Another form of E—young male, 7 mm. 
HOH UO 
All the figures are magnified and photographed on 
the same scale. 
