The Isopods of Abyssal Depths in the Atlantic Ocean

and peraeon continuous. Flagellum of first antenna
with four articles. Postero-lateral angles of pleon
extending beyond medial margin. Dorsum of pleon
smooth. First article of first antenna not concealed by
cephalon. Uropods not extending to posterior border
of pleon.
Measurements: Length 2.6 mm, width pleon 0.8
mm.
Type locality: South Atlantic, L.G.O. Biotrawl No.
51, one female, cat. no. 1-34.
Distribution: Known also from L.G.O. Biotrawl
No. 52, one male, two female, cat. no. 1-35.
Affinities: This species appears most closely related
to A. rostratus, from which it differs mainly in having
the lateral margins of the peraeon and pleon continuous.
Antennuloniscus rostratus, new species
Figure 14 G-J
Synonyms: None.
Diagnosis: Frontal border convex with a sharp
upcurved spine medially. Lateral margin of pleon
peraeon continuous. Flagellum of first antenna with
five articles. Postero-lateral angles of pleon extending
beyond medial margin. Dorsum of pleon smooth.
First article of first antenna not concealed by cephalon.
Uropods not extending to postero-lateral border
of pleon.
Measurements: Holotype female length 3.3 mm.,
width pleon 0.55 mm.
Type locality: South Atlantic, L.G.O. Biotrawl No.
52, three females, cat. no. 1-36.
Distribution: Known only from type locality.
Affinities: The long and broad rostrum of this
species is distinctive; also the lateral border of the
sixth peraeonal somite is produced outward from that
of the seventh.
Antennuloniscus ornatus, new species
Figure 14 K-M
Synonyms: None.
Diagnosis: Frontal margin of cephalon convex.
Pleonal and peraeonal lateral margins continuous.
Flagellum of first antenna with six articles. Posterolateral
angles of pleon not projecting beyond medial
pleonal margin. Dorsum of pleon with ridge near
each lateral margin. First article of first antenna not
concealed by cephalon. Rami of male first pleopod
separated at apex. Uropods extending to posterior
margin of pleon. Antero-lateral borders of fourth
peraeonal somite not produced forward.
Measurements: Holotype male length 1.9 mm.,
width pleon 0.8 mm.
Type locality: South Atlantic, L.G.O. Biotrawl
No. 47, two males, cat. no. 1-33.
ABYSSAL ATLANTIC ISOPODS 109
Distribution: Also known from L.G.O. Biotrawl
No. 201, one male, cat. no. 1-223.
Affinities: The dorsal elevation of the cephalon
and the carinae on the pleon separate this species
from all the others. Its nearest known relative is
possibly A. dimeroceras (Barnard), a species also known
from the South Atlantic.
Genus: HYDRONISCUS Hansen
Type species: Hydroniscus abyssiHansen, 1916
Diagnosis: Haploniscidae with the pleon and last
three peraeonal somites fused into a solid piece.
Uropoda extremely reduced or entirely absent.
Third article of peduncle of first antenna longer than
wide but lacking the angulate projection characterizing
Haploniscus.
Composition: The genus contains three species, two
from the North Atlantic and one from the South
Atlantic. They are markedly different from one
another. According to Wolff (1960), Birstein has
found a species of Hydroniscus in the North Pacific.
A KEY TO THE SPECIES OF
HYDRONISCUS
1. Uropoda present ornatus, n. sp.
1. Uropoda absent 2
2. Rostrum pointed abyssi Hansen
2. Rostrum blunt guadrifrons, n. sp.
Hydroniscus ornatus, new species
Figure 15 A-H
Synonyms: None.
Diagnosis: Hydroniscus with distinct pointed postero-lateral
angles projecting laterally from pleon.
Single-jointed uropoda present, not extending beyond
posterior margin of pleon. Flagellum of first antenna
with four articles, that of second with 12 articles.
Measurements: Holotype male length 3.5 mm.,
width pleon 1.7 mm., allotype female length 4.0 mm.,
width pleon 1.75 mm.
Type locality: South Atlantic, types plus three female
paratypes from L.G.O. Biotrawl No. 51, cat. no.
1-43.
Distribution: Known also from L.G.O. Biotrawl
No. 217, one female, cat. no. 1-211, in the South
Atlantic.
Affinities: Ornatus is possibly closely related to
guadrifrons, from which it differs in having a rounded
rostrum and in having very pronounced posterolateral
angles on the pleon.
Hydroniscus quadrifrons, new species
Figure 15 I-M
Synonyms: None.
Diagnosis: Hydroniscus with minute postero-lateral
angles on pleon. Quadrate single-jointed uropods