29 



palp unisetose ; posterior maxilla 1 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 

 <!' legs in female provided with well-developed natatory exopodites; 2nd pair 

 strongly built, conspicuously fossorial in character, and having the ischial and 

 meral 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. 



Remark*. 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 VaunthompsoniiddB 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 

 DiaxtylidcK. By the total absence of the telson, the family Leuconidw 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. I. LeUCOn, Kr0jer, 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. Car-apace 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 antennas of moderate size, with the pe- 

 duncle not geniculate, inner flagellum quite rudimentary, knob-like. Inferior 



