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G.O. Sars, 1870

Description
Generic features. Statocyst in the uropods small. Pleopods of female reduced, rudimentary; of male variable. Exopod of uropod with proximal portion of outer margin naked, marked distally by one or two spines and an incipient articulation. Marsupium with seven pairs of oostegites. Telson cleft.
—Species. Body robust. Carapace with the front margin produced into a tridentate rostral plate, the central tooth slightly in advance of the lateral, rostral plate not extending beyond the eyestalks, antero-lateral corners of the carapace acutely produced.
Eyes large, pyriform, cornea wider than the stalk. Eye pigment, golden to red-brown.
Antennal scale about twice as long as the antennular peduncle, about four times as long as broad, outer margin slightly curved, entire, terminating in a spine beyond which the apex of the scale does not extend, apex truncate, a small oblique distal suture present; a prominent spine on the outer distal corner of the sympod.
Uropod with endopod slightly longer than the telson with 2 spines near the proximal end of the inner margin; exopod one-quarter longer than the endopod and considerably broader; with truncate apex, proximal third of outer margin naked, this portion terminated by 2 spines from which an incomplete suture runs obliquely across the exopod; statocyst small.
Telson longer than last abdominal somite, three times as long as broad at base; apex cleft to one-fourth of the entire length of the telson, margins of cleft slightly diverging, armed with a continuous row of saw-like teeth; no proximal dilation to the cleft; no plumose setae.

Size
Length of adults of both sexes 30 mm.

Depth range
Hyperbenthic; recorded depths range 550-2500 m.

Remarks
Boreomysis tridens is easily distinguished from all other species of the genus by the tridentate rostral plate. Other characters which serve to distinguish this species are:

- the large eyes,
- the form of the antennal scale and its greater length compared with the antennular peduncle,
- the lack of a subchelate termination to the endopod of the second thoracic limbs,
-the division of the propodus of the third to the eight thoracic limbs into two subsegments,
- the absence of a proximal dilation in the cleft of the telson,
- the relatively great extent of the unarmed portion of the outer margins of the exopods of the uropods and the fact that only the third pleopod of the male has a modified distal end to the exopod.

Distribution in the North Sea
Northern North Sea, W off Norway.

World distribution
NE North Atlantic: 28-70°N; shelf to bathyal. Distribution is confined to the North Atlantic Ocean (NE North Atlantic 28-70°N) in suitable depths on the Atlantic slope of Europe from the Bay of Biscay to Norway. It is also known form Greenland and Southwards off the Atlantic slope of North America as far as the coasts of New England.

[After Tattersall and Tattersall, 1951]

Boreomysis tridens