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palp unisetose; posterior maxillse very small, without any setse inside. Branchial 
apparatus with a limited number of digitiform gill-elements. 3rd pair of maxilli- 
peds comparatively short, with the basal joint dilated distally, and carrying on 
the projecting outer corner a number of very strong setse. The 3 anterior pairs 
of legs in female provided with well-developed natatory exopodites; 2nd pair 
strongly built, conspicuously fossorial in character, and having the ischial and 
literal joints coalesced. Adult male with very fully developed natatory exopodites 
on all the legs except the last pair, but with only 2 pairs of pleopoda. Uropoda 
with both rami Inarticulate, the inner spinulose, the outer setiferous. Telson 
absent. 
Remarks .—This family comprises forms of rather different external ap¬ 
pearance, but very closely agreeing in some of the anatomical details. The struc¬ 
ture of the oral parts in particular, is very characteristic, and rather unlike that 
found in most other Cumacea. The presence in the female of well-developed 
natatory exopodites not only on the 2 anterior pairs of legs, but also on the 
3rd pair, is another character by which the present family is distinguished, only 
the family Vaunthompsoniidce agreeing with it in this respect. But, whereas in that 
family, the male has 5 well-developed pairs of pleopoda, the number of these ap¬ 
pendages in the present family is limited to 2 pairs only, as in the family 
Diastylidce. By the total absence of the telson, the family Leuconidce is, how¬ 
ever, at once distinguished both from the last-named family and from the 2 im¬ 
mediately preceding it. 
We only know at present of 3 genera belonging to this family, and all 
of these are represented in the fauna of Norway, and will be treated of below. 
Gen. 1 . LeilCOn, Kr 0 yer, 1846. 
Generic Characters. —Body, as a rule, slender, with the anterior division 
more or less compressed. Carapace in female with a serrated crest along the 
middle, pseudorostral projection prominent, and defined from the antero-lateral 
corners by an angular cleft, lower edges of carapace bent in the middle, with 
the anterior half serrate. Carapace of male generally without any dorsal crest, 
and having both the pseudorostral projection and the antero-lateral corners blunted. 
Tail slender and very|mobile. Superior antennre of moderate size, with the pe¬ 
duncle not geniculate, inner flagellum quite rudimentary, knob-like. Inferior 
