inside, and a row of ciliatod setae outside, its distal joint scarcely half as long 

 as the proximal one, and carrying on the tip a remarkably strong spine accom- 

 panied by a long seta; outer ramus densely clothed with ciliated setse. Body 

 without any pigmentary ornament, of whitish colour, changing in female to pale 

 yellow. Length of adult female reaching to 12 mm., of male to 10 mm. 



l\i'iiuirl-x. This form was first described by Kr0yer as a species of 

 ('n)ini. but was subsequently separated as the type of his genus Leucon. It is 

 the largest of the Norwegian species, and moreover easily recognizable by its 

 very slender form and the nose-like, upturned pseudorostral projection. In the 

 young, not yet sexually mature state, the male does not exhibit any pronounced 

 difference in its general appearance from the female, the form and armature of 

 the carapace being the very same in both. It is quite otherwise when the male 

 has reached full sexual maturity. In this state, on account of the peculiar 

 transformation of the carapace, it looks so very different from the female that 

 even in quite recent times, its true relation lias been wholly misapprehended. In- 

 deed, there cannot be any doubt that the form recently recorded by Mr. Walker 

 as LeiicoHojixi* Dixifrr is nothing else than the adult male of the present species. 

 As will be shown farther on, an altogether analogous transformation of the male 

 is proved to take place in all our other species of this genus. 



Occurrence. This is one of the commonest Cumacea of the Norwegian 

 coast, occurring very frequently throughout its whole length, from the Christiania 

 Fjord to Vadso. in depths from 30 to 100 fathoms, soft muddy bottom. The 

 adult males seem, only at certain seasons, to appear in some numbers. They are 

 extremely agile, swimming about with great rapidity by the aid of the very fully 

 developed exopodites, and the likewise well-developed 2 pairs of pleopoda. The 

 females, on the other hand, lead a much more stationary life, mostly dwelling on 

 the bottom, in which they are able to bury themselves with great dexterity. 



Distribution. British Isles (Norman), Kattegat (Meinert), Heligoland 

 (Ehrenbaum), Greenland (Hansen), the Kara Sea (Hansen), Atlantic coast of 

 North America (Verrill). 



2. Leucon nasieoides, Lilljeborg. 

 (PL xxm.) 



Leucon nasieoides, Lilljeborg, Ofvers. af Vet. Akad. Forh. 1855, p. 122. 



Specific Characters. Body of female considerably shorter and more robust 

 than in the type species, with the tail scarcely exceeding in length the anterior 

 division. Carapace rather deep, and about the length of the exposed part of 



