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. The 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 ; tlie 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 

 antenna?. 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. This mark 

 reminds one of the similar depressed mark found on the head in 

 many species of Idotea. The inferior mai'gin 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 

 aqiiaticus 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 douce de Norvege, p. 94, pi. viii., fig. 9. 



