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of the flagellum are “auditory cilia,” one or two on each joint. 
These are very small but are of the usual shape being similar to 
those of Asellus aquations as figured by Sarsv* 
Lower antenna . (Plate xxiii., fig. 3.) — The peduncle consists of 
five joints and is considerably shorter than the flagellum. The 
first joint is short, the second, third, and fourth, are subequal in 
length, the fifth about half as long again as the fourth, but nar- 
rower ; the third is rounded above and bears two small tufts of 
sete, small tufts are also present on the fourth and fifth joints. 
The first joint of the flagellum is longer than the succeeding, the 
next three or four are usually short, the others subequal in length 
but gradually becoming narrower ; towards the end they also 
increase a little in length. Each joint bears one or two small 
sete above and below at the distal end. 
The upper lip (Plate xxiii., fig. 4) is large and strong, regularly 
rounded distal ly, the centre being slightly produced. It bears a 
number of short sete thickly set together and converging towards 
the centre of the distal margin. 
The mandibles (Plate xxiii., figs. 5 and 5a) are large and powerful. 
In a side view of the head the basal joint can be easily seen ex- 
tending along the anterior portion of the lower margin of the head; 
just below the base of the lower antenna it gives off the three- 
jointed palp which extends anteriorly beneath the antenna?, from 
this point the basal portion of the mandibles extends downwards 
and forwards and curves inwards to form the cutting edge. From 
the inner surface arises the large and powerful molar tubercle 
which extends obliquely upwards and inwards almost at right 
angles to the portion of the mandible from which it springs, until 
it reaches the median line and meets the molar tubercle of the 
other side. The two mandibles are of the same size and general 
appearance but differ in a few details. It will be convenient to 
describe the left mandible first. 
The cutting edge of the left mandible consists of two separate 
processes one inside the other ; the outer one consists of four 
sharp strong teeth, brown in colour, and the inner one of three 
similar teeth. Figure 5a of plate xxiii., shows the ends of these 
two processes as seen from the inside. Within these two processes 
is another, the chitinous integument of which is less strong and 
thickened ; it is rather slender but expands somewhat distal ly 
and extends inwards to the median line so that the end of it is in 
a line with the cutting edge of the mandible and the end of the 
molar tubercle ; the end is crowned with about 15 to 20 sharp 
strong sete which project radially from the end. Between the 
base of this process and that of the molar tubercle are four or 
five plumose or pectinated sete nearly as long as the last mentioned 
* Crustaces d’eau douce de Norvege, plate viii., fig. 19. 
