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G.O. Sars, 1877

? Generic features. The structure of the thoracic limbs is are markedly more unguiculate than in any other genus of mysids. The claw is surrounded by a stiff brush of long, peculiar, microscopically spinose setae, giving a very characteristic appearance which can readily be recognised.
Carapace rather short. Antennal scale with outer margin naked and ending in a thorn, apex obliquely truncate. Eyes well developed.
Labrum much longer than broad, produced anteriorly into very long process.
Mandible molar tubercle feebly developed; maxillae narrow, lobe from the third segment feebly cleft.
Proximal segment of exopod of uropod armed with spines but no setae. Endopod of uropod undivided.
Thoracic endopod markedly unguiculate. First pair of thoracic limbs slender, gnathobasic lobe on the second segment only of the endopod or entirely absent; third pair of thoracic limbs with the endopod not elongated and dactylus and claw well developed; pleopods of female reduced to small, simple, linear plates with a few plumose setae. Exopod of uropod with distinct distal suture. Telson entire.
Three pairs of oostegites in the female.
? Species. Body long and slender. Exopod of uropod with a distinct distal suture. Antennal scale with outer margin naked and ending in a thorn. Telson entire. Thoracic endopods markedly unguiculate.
Carapace very short, rostrum acutely triangular, reaching nearly half-way along the first joint of the antennular peduncle.
Antennal peduncle more than twice as long as eye; first segment equal in length to the other two together, third segment in female armed with one plumose seta on inner margin in addition to the group at inner distal corner; outer flagellum much longer and stronger than inner.
Antennal scale sub-rhomboidal, extending nearly to distal end of antennular peduncle, about three times as long as broad, outer margin straight.
Mandibles with molar tubercles feebly developed.
Eyes large, much shorter and thicker than in S. armata, cornea occupying nearly half the organ, pigment black.
Thoracic limbs somewhat shorter and more robust than in S. armata; carpus much shorter than propodus.
Telson somewhat shorter and broader than in S. armata, margins armed near the base with four strong graduated spines which are separated from those more distally placed by a bare space. In distal portion there are three to five small spines occupying intervals between larger spines; apex with three equal spinules and two setae between the two large terminal spines which are set rather far apart.
Uropods shorter and broader than in S. armata, exopod only slightly longer than telson, less than five times as long as broad, distal segment about onefourth of its entire length, not twice as long as broad, outer margin armed with 8 to 10 spines which increase in size distally; endopod shorter and narrower than exopod, inner margin armed with a continuous row of spines mostly with groups of smaller spines between and the distal three or four spines very large without intermediates.

Length of female 10 mm; male 11 mm.

Siriella clausi is not found at greater depths than 30 metres. During the breeding season migrating to the upper waters during the hours of darkness.

This species closely resembles S. armata in general form, but can at once be distinguished from it by its shorter, less acute rostrum.
Siriella clausi is definitely a littoral species, living in shallow water, in rock pools and among weeds on gravel, rocky and sandy bottoms.

Distribution in the North Sea:
All coastal parts of the North Sea.

World distribution:
Atlantic Ocean: 33-58°N, Mediterranean, Black Sea; coastal, shelf.

Siriella clausii