The Isopods of Abyssal Depths in the Atlantic Ocean

pleopods each with simple rounded apex bearing
three setae.
Measurements: Male holotype length 3.1 mm.,
width pleon 0.2 mm. Allotype length 4.1mm., width
pleon 0.4 mm.
Type locality: North Atlantic, Mediterranean,
L.G.O. Biotrawl No. 76, holotype and allotype, cat.
no. 1-105.
Distribution: Also taken at L.G.O. Biotrawl No.
95, one male fragment, cat. no. 1-106, Caribbean.
Affinities: This species appears most closely related
to H. insignis Hansen, from which it differs in having
much smaller lateral body spines.
Haplomesus insignis Hansen
Figure 20 N-P
Synonym: Haplomesus insignis Hansen, 1916, p. 63,
PL V.
Diagnosis: Haplomesus with massive lateral spines
on peraeonal somites 1 and 4 only. Pleon with pronounced
postero-lateral angles, dorsum without
stout setae or carinae. Uropoda longer than wide
and extending beyond posterior margin of pleon.
Measurements: Male 4.5 mm. long (Hansen, op.
cit.).
Type locality: North Atlantic, Davis Strait, Ingolf
Station 36, latitude 61° 50' N., longitude 56° 21' W.,
1435 fathoms (2702 meters), temperature 1.5° C.
(Hansen, op. cit.).
Distribution: Taken also by Vema at L.G.O.
Biotrawl No. 234, one anterior fragment, cat. no.
1-69. Reported by Gorbunov (1946, p. 123) from
698 meters in the Arctic.
Affinities: This species appears to be related to
H. tropicalis, but has much more massive lateral body
spines.
Haplomesus ornatus, new species
Figure 21 A-B
Synonyms: None.
Diagnosis: Haplomesus with lateral spines on
peraeonal somites 1 and 3 only. Peraeon strongly
tuberculate. Apex of pleon truncated, posterolateral
angles lacking, dorsum with paired medial
carinae. Apex of male pleopods each with blunt
lateral process and six setae.
Measurements: Holotype male length 6.8 mm.,
width pleon 0.9 mm.
Type locality: South Atlantic, L.G.O. Biotrawl No.
18, one male and one male fragment, cat. no. 1-49.
Distribution: Also from L.G.O. Biotrawl No. 52,
one male, cat. no. 1-47, and L.G.O. Biotrawl No. 214,
two fragments, cat. no. 1-78.
Affinities: This species is near to the North Atlantic
/. quadrispinosis (G. O. Sars), but has carinae on the
dorsum of the pleon which the latter lacks.
ABYSSAL ATLANTIC ISOPODS 121
Haplomesus bifurcatus, new species
Figure 21 C-I
Synonyms: None.
Diagnosis: Haplomesus with lateral spines on
peraeonal somites 1 and 3 only. Peraeon strongly
tuberculate. Pleon without pronounced posterolateral
angles; apex incised, dorsum with mid-central
swelling with deep pit. Uropoda minute, as wide as
long and not extending to apex of pleon. Male first
pleopods each with minute lateral projections and
six setae.
Measurements: Holotype male length 5.0 mm.,
width pleon 0.7 mm.
Type locality: South Atlantic, L.G.O. Biotrawl No.
12, holotype and one male fragment paratype, cat. no.
1-48.
Distribution: Known only from type locality.
Affinities: This species resembles H. ornatus, but the
indentation of the posterior border of the pleon distinguishes
it.
Genus: HETEROMESUS Richardson
Synonyms: Heteromesus Richardson, 1908a, p. 81;
— Hansen, 1916, p. 66; — Wolff, 1956, p. 141.
Type species: Ischnosoma thomsoni Beddard.
Diagnosis: Ischnomesidae with the third article of
the first antenna minute. Pleon with a single somite
only. Uropoda with a single article. Sixth and
seventh peraeonal somites fused with pleon.
Composition: Thirteen species of this genus are
known. Additionally, one new one is described in this
paper. They are bathyal to abyssal in depth distribution
and are known from only the Arctic, the
North Atlantic (including the Caribbean), and the
North Pacific (four species).
1. thomsoni (Beddard)
2. spinosus (Beddard)
3. greeni (Tattersall)
4. spinescens Richardson
5. granulatus Richardson
6. dentatus Hansen
7. longiremis Hansen
8. schmidtii Hansen
9. frigidus Hansen
10. similis Richardson
11. gigas (Birstein)
12. scabriusculus (Birstein)
13. robustus (Birstein)
Least
3750
1829
364
2155
713
1505
698
956
698
—
6560
5450
5450
Greatest
3750
1829
700
3337
3235
1505
2707
956
1435
2995
8430
5450
5817
A KEY TO THE SPECIES OF
HETEROMESUS a
It is not easy to provide a key to the species of Heteromesus
because entire specimens are not yet known for all
the species. The following key therefore is incomplete but
probably useful.