The four pairs of legs now described form an anterior series, 
differing considerably in form from the remaining three which 
are similar to one another and form a posterior series, the members 
of which increase in size posteriorly. 
The fifth thoracic leg (third pereiopod) (See Plate xxv., fig. 3) 
is slightly longer than the fourth. The basos is produced poster- 
iorly into a thin flat expansion with convex margin fringed with 
long setre — much in the same way as in many Amphipoda. The 
ischios is large, about two thirds as long as the basos and is sub- 
rectangular, though expanding somewhat distally; it bears tufts 
of stout seta' on both margins. The meros , carpus , and propodos 
are all similar and suhrectangular, but each is longer and narrower 
than the preceding, they all bear numerous tufts of stout spiniform 
seta* on both margins. The meros has the postero-distal angle 
slightly produced as in many Amphipoda. 
The sixth and seventh thoracic legs ( fourth and fifth pereiopoda) 
are similar to the fifth but are much larger, the various joints are 
all similar to the corresponding joints of the fifth leg, but they 
bear a greater number of setae, and the setae themselves are stouter, 
the basos has the posterior margin more produced and more con- 
vex and the dactylos is longer and more slender. 
Figure 3 of Plate xxv., shows the last (seventh) leg, but will 
do almost equally well to represent the general appearance of the 
fifth and sixth. 
It is worthy of note that although these thoracic legs are very 
Aiuphipodan-like in general appearance, they all have the ischios 
large and well developed instead of being small and very short as 
in almost all the Amphipoda, and that although the first pair of 
logs has a well developed subchelate hand, the second pair is quite 
simple, while as a general rule both pairs are more or less sub- 
chelate in the Amphipoda. The resemblance of these legs to 
those of the Amphipoda is therefore more superficial than real, 
and a comparison of my figures with those given by Sars of Asellus 
aquations will show that there is a close general resemblance 
between the two. 
I have called all these legs “thoracic legs 5 ' instead of speaking 
of the first two pairs as “ gnathopoda 55 and the remainder as 
“ pereiopoda as is usually done in the Amphipoda. This would 
have made an arbitrary and misleading distinction between the 
second and third pairs which are precisely similar to each other, 
while it would probably have led to confusion if I had spoken of 
them all as u pereiopoda/ 5 as those of the last pair would then be 
the seventh pereiopoda, while if the plan adopted with the Amphi- 
poda were adhered to they would be the fifth pereiopoda. It 
seems to me a pity however that the term “ pereiopoda 37 has not 
been applied to all the appendages of the pereion both in the 
Amphipoda and Isopoda, leaving the term “gnathopoda 5 ' a special 
