The Isopods of Abyssal Depths in the Atlantic Ocean

184 ROBERT J. MENZIES
Genus: THAMBEMA Stebbing
Type species: Thambema amicorum Stebbing, 1912,
p. 42; 1913, p. 237.
Synonyms: Thambema Stebbing, 1912, p. 42; —
1913, pp. 237-239.
Diagnosis: Same as for the family.
Composition: Monotypic.
Thambema amicorum Stebbing
Figure 63
Synonyms: Thambema amicorum Stebbing, 1912, p.
42; — 1913, pp. 237-239, PI. 26.
Diagnosis: Thambema with frons of cephalon convex
and even, pleon without spines or serrations, apex
evenly rounded. Apex of male first pleopod pointed
without lateral expansions. Second article maxillipedal
palp wider than third, fourth and fifth very
narrow.
Measurements: About 8 mm. (Stebbing, 1913, p.
239).
Type locality: North Atlantic, west of Donegal,
Porcupine Station 19, latitude 54° 53' N., longitude
10° 56' W., 2486 meters.
Distribution: Known only from type locality. Not
captured by Vema.
Genus: MESOSIGNUM, new genus
INCERTAE
Synonyms: None.
Type species: Mesosignum kohleri, new species.
Diagnosis: Paraselloidea with free cephalon. All
peraeopods ambulatory. Mandible with a triarticulate
palp and a toothed incisor, setal row present, lacinia
present, molar tapering to a flat setiferous point.
Uropods uniramous, with peduncle, insertion lateral.
Maxillipedal palp with narrow articles all of similar
width and one-half the width of endite. First male
pleopod rounded at apex. Cephalon without eyes or
eye stalks. First antenna about twice the length of
cephalon. Peraeonal somites, except first provided
with spine-like lateral expansions. Coxal plates
visible in dorsal view only on somites 5—7 inclusive.
Anus separated from branchial cavity.
Remarks: The mandibles of this genus resemble
Pleurosignum (Pleurogonidae), whereas the maxillipedal
palp is like the Dendrotioniidae. The epimeral
plates are like Austrosignum (Pleurogonidae); whereas
the uropods are unique. Assignments of the genus to
an existing family is impossible and the establishment
of a new one seems not warranted at this time.
Composition: The genus Mesosignum contains two
species, both abyssal and bathyal; both are new and
from the Caribbean.
Figure 63. Thambema amicorum Stebbing. A:
maxillipeds; B: dorsal view cephalon; G:
third pleopod; D: mandible; E: second
pleopod; F: first gnathopod; G: dorsal view
pleon; H: first pleopod.
SEDIS
A KEY TO THE SPECIES OF
MESOSIGNUM
Apex of pleon rounded kohleri, n. sp
Apex of pleon bifurcated by stout spine-like
projections usheri, n. sp
Mesosignum kohleri, new species
Figure 64 A-G
Synonyms: None.
Diagnosis: Mesosignum with apex of pleon evenlv
rounded, postero-lateral projections lacking. Frons of
cephalon pointed. Antero-lateral spine of seconc
somite only twice the length of postero-lateral spine
and not extending forward beyond peduncle of first
antenna. Apex of male first pleopod rounded with
ten marginal setae.
Measurements: Male holotype length 2.5 mm.,
width pleotelson 0.35 mm., allotype gravid length 2.3
mm., width pleotelson 0.30 mm.
Type locality: North Atlantic, Caribbean, L.G.O.
Biotrawl No. 96, types plus one male, three female
paratypes, cat. no. 1-82.
Distribution: L.G.O. Biotrawl No. 94, three males,
two females, two juveniles, one fragment, cat. no.
1-84; No. 95, three males, cat. no. 1-83; No. 97, one
female, cat. no. 1-85; No. 98, two males, cat. no. 1-86.