2. THE SPECIES O F KRILL
37
2. The two species, N . diflcilis and N . megalops, can only be disSee Boden
tinguished by examination of the petasma in males.
et al. (1955), Boden (1954).
1 b. Second pair of thoracic legs with long spines from terminal segment only.
Third and fourth thoracic legs with only two segments beyond knee, and
fifth and sixth with only one.
3a. Lower part of eye much smaller than upper
. . . .
tenella
3b. Lower part of eye about same size or larger than upper.
4s. Rostrum straight, reaching to or beyond anterior margin of eyes,
variable in male. Two species very similar, probably females
identified by examination of thelycum (Einarsson, 1942 ; Ruud,
1936) . . .. .. .. . . microps and atlantica
4b. Rostrum does not reach anterior margin of eyes or just reaches it.
5. Two species distinguishable by form of petasma (Hansen,
1916; Boden et al., 1955) . . ..
lobata and gracilis
Genus Nematobrachion
la. Rostrum present, lateral denticle on carapace, and dorsal processes on
some abdominal segments present.
2a. Mid-dorsal acute process present on posterior margins of third, fourth
and fifth abdominal segments
. . .. .. . . jlezipes
2b. Mid-dorsal acute process absent on third abdominal segment ; middorsal process present on fourth and fifth segments flanked on either
side by a smaller acute process . . .. ..
sexspinosum
lb. Rostrum absent, no lateral denticles on carapace, and no dorsal processes
on abdominal segments . . .. .. .. .. . . boopis
Genus Stylocheiron
One species of doubtful validity is not included in the k e y ; it is
8. armatum Colosi, 1917, which is probably synonymous with S. carinaturn
Sam, 1885.
la. Penultimate segment of the elongated third pair of thoracic legs with
lateral setae only. Upper eye with numerous (7-8), slightly enlarged
crystalline cones in transverse row. Thorax slender. . . . carinaturn
lb. Penultimate segment of the elongated third pair of thoracic legs terminates in false chela. Upper eye with conspicuous, elongate, crystalline
cones in transverse row.
2a. Upper eye with from two to six crystalline cones in transverse row.
3a. Upper eye with four or five, rarely six, crystalline cones in row
aflne
37
2. The two species, N . diflcilis and N . megalops, can only be disSee Boden
tinguished by examination of the petasma in males.
et al. (1955), Boden (1954).
1 b. Second pair of thoracic legs with long spines from terminal segment only.
Third and fourth thoracic legs with only two segments beyond knee, and
fifth and sixth with only one.
3a. Lower part of eye much smaller than upper
. . . .
tenella
3b. Lower part of eye about same size or larger than upper.
4s. Rostrum straight, reaching to or beyond anterior margin of eyes,
variable in male. Two species very similar, probably females
identified by examination of thelycum (Einarsson, 1942 ; Ruud,
1936) . . .. .. .. . . microps and atlantica
4b. Rostrum does not reach anterior margin of eyes or just reaches it.
5. Two species distinguishable by form of petasma (Hansen,
1916; Boden et al., 1955) . . ..
lobata and gracilis
Genus Nematobrachion
la. Rostrum present, lateral denticle on carapace, and dorsal processes on
some abdominal segments present.
2a. Mid-dorsal acute process present on posterior margins of third, fourth
and fifth abdominal segments
. . .. .. . . jlezipes
2b. Mid-dorsal acute process absent on third abdominal segment ; middorsal process present on fourth and fifth segments flanked on either
side by a smaller acute process . . .. ..
sexspinosum
lb. Rostrum absent, no lateral denticles on carapace, and no dorsal processes
on abdominal segments . . .. .. .. .. . . boopis
Genus Stylocheiron
One species of doubtful validity is not included in the k e y ; it is
8. armatum Colosi, 1917, which is probably synonymous with S. carinaturn
Sam, 1885.
la. Penultimate segment of the elongated third pair of thoracic legs with
lateral setae only. Upper eye with numerous (7-8), slightly enlarged
crystalline cones in transverse row. Thorax slender. . . . carinaturn
lb. Penultimate segment of the elongated third pair of thoracic legs terminates in false chela. Upper eye with conspicuous, elongate, crystalline
cones in transverse row.
2a. Upper eye with from two to six crystalline cones in transverse row.
3a. Upper eye with four or five, rarely six, crystalline cones in row
aflne