CRUSTACKA MALACOSTRACA. IV. 



Family Leuconidae. 



As to geographical distribution this moderately large family is extremely different from the Bodo- 

 triidae. While, as pointed out by Zimmer, the latter family has a greater number of forms in the tropical 

 than in the northern or southern temperate seas, and only a couple of representatives, both deep-sea forms, 

 within the "Ingolf" area, no member of the Leuconidae has been found in the tropical seas, while a rather good 

 number of forms inhabit the boreal-arctic and the subantarctic and antarctic seas. Three well-known genera 

 are represented in the "Ingolf" area. 



LeUCOn Kroyer. 



Of this genus the "Ingolf" and the "Thor" together has secured a large number of forms, viz. all 

 species taken in the area in question by earlier expeditions excepting L. longirostris G. O. Sars, furthermore 

 all species captured according to Sars at Norway and according to Caiman at Great Britain and Ireland, 

 finally 5 new species. The result is that no less than 14 species (L. longirostris G. O. S. included) are known 

 from our area, an astonishingly high number of this genus. A few species are found in rather low water or 

 in moderate depths; several are mainly from depths between 200 and 600 fathoms, and some are real deep- 

 sea forms. 



The following arrangement, though partly somewhat artificial, may be of some use for the student 

 of this somewhat difficult genus. 



A Species with an extremely long, freely projecting branchial siphon. 



3. Leucon siphonatus Calm. 



11905. Leucon siphonatus Caiman, Fisheries, Ireland, Sci. Invest. 1904, I, p. 19, PI. I, figs. 2 — 4. 

 1906. Caiman, Mitth. Zool. Stat. Neapel, 17. Bd. 4. Heft, p. 416, PI. 27, fig. 9. 



1913. Stebbing, Das Tierreich, 39. Lief. p. 64. 



The single specimen is a female with marsupium, ^,.j mm. long. It agrees on the whole well with 

 Caiman's description and figures; especially the somewhat curved pseudorostrum with its very concave 

 lower margin is most characteristic. On the anterior part of one of the sides of the carapace I have counted 

 at least 16 teeth. More than the anterior half of the lower margin of the carapace is serrated, but most of 

 the teeth are difficult to see because they are almost subparallel with the margin. Pseudorostrum has 5 teeth 

 on the upper margin, but the single tooth anteriorly on the dorsal crest in Caiman's Irish specimen does not 

 exist in my specimen. The upper part of the anterior margin of third free thoracic segment has several fine 

 teeth, but no armature could be perceived on any of the other segments. In the antennula; the third joint 

 of the peduncle is nearly as long, but only half as thick, as the second ; it is nearly half as long again as the 

 external flagellum. In second pair of legs the merus is very thick and short, the caqjus very slender and 

 elongated, almost twice as long as the merus and a little longer than the two distal joints combined. 



