153 
male. The body is broadest at the third segment of the pereion 
and thence the breadth gradually decreases posteriorly, the pleon 
being somewhat laterally compressed. The depth of the body in 
the pereion is about equal to the breadth, but in the pleon, owing 
to the pleura being produced downwards, the depth is about twice 
the breadth. Tile dorsal surface is regularly rounded and very 
convex, the ventral surface of the pereion is almost flat so that 
the pereion is pretty regularly sub-cylindrical; the pleon presents 
the appearance of a semi-cylinder laterally flattened. 
Head , (Plate xxiii., fig. 1.)— The dorsal surface of the head is 
very convex and curves downwards anteriorly making the outline 
as seen in a lateral view subtriangular. The anterior margin is 
deeply emarginate in the centre behind the bases of the superior 
an ten me. A clearly marked depression runs across the dorsal 
surface near the posterior margin and extends down the sides, 
running out into the posterior margin of the head. Tin's mark 
reminds one of the similar depressed mark found on the head in 
many species of Idotea. The inferior margin is nearly straight, 
the notch serving for the articulation of the base of the mandible 
being small and shallow, and by no means so distinct as in Asellus 
aquations as described and figured by Sars.* In front of the eye 
and a little below it is a deep cleft in the anterior margin of the 
head, and a slight depression extends backwards from this cleft 
below the eye. 
Eyes . The eyes are situated laterally on the portion of the 
head which is slightly produced on each side at the bases of the 
antenna?. They are of moderate size, round in shape and consist 
of about 20 lenses arranged fairly regularly in circular rows and 
separated from one another by distances about equal to the 
diameter of the lenses. 
Pereion , (Plate xxiii., fig. 1.) — The first segment of the pereion 
is closely attached to the head and appears to be capable of little 
or no independent motion, though the division mark between the 
two is very distinct. It is worth while to recall the fact that this 
is also the case with some species of Idotea , in some of which as 
Idotea elongata , Miers, the union of the head with the first seg- 
ment of the pereion has gone so far that the line of division is 
almost obliterated. The head and first segment of the pereion 
are confluent also in Apseudes and in Tanais . When viewed from 
above the first segment is seen to be very short (narrow) in the 
centre, and to have the anterior and posterior margins both con- 
cave for the reception of the head and second segment of the 
pereion respectively. In side view the segment widens inferiorly 
and is somewhat produced anteriorly so as to impinge closely upon 
the head. 
* Crustaces d’eau douco de Norv^ge, p. 94, pi. viii, 3 fig. 9. 
