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(G.O. Sars, 1864)

Description
Generic features. Carapace with anterior margin drawn out into a short, triangular rostrum, bluntly rounded; antero-lateral angels rounded; posterior border emarginate. Antennule similar in form to that of Leptomysis. Antennal scale about one-third to one-half as long again the antennular peduncle, setose all round; small distal suture present; prominent spine on outer distal corner of the sympod.
Eyes very large, with small triangular eyestalks; somewhat flattened dorso-ventrally, set close together and extending only slightly beyond the lateral margins of the carapace.
First thoracic limb with endopod very robust and densely furnished with strong barbed setae. Pleopods of the male well developed, biramous and many-segmented; pleopods of the female rudimentary, in the form of slender, unsegmented plates. Uropod slender, endopod shorter than the exopod and armed in the region of the statocyst with from two to five graduated spines.
Telson with small apical incision, the incision armed at its proximal end with at least one pair of long, plumose seta, there are ca. 20 marginal spines.
Marsupium consisting of three pairs of incubatory lamellae.
— Species. General form somewhat robust. Carapace produced anteriorly into a short triangular rostrum, slightly rounded at the apex and extending to less than half the length of the first segment of the antennular peduncle; antero-lateral angles rounded; emarginate posteriorly leaving the whole of the eighth and seventh and part of the sixth thoracic somites exposed in dorsal view.
Antennule with the first and third segments subequal; third segment armed along its inner border and at the inner distal angle with a number of strong plumose setae.
Antennal peduncle: almost as long as the antennular peduncle; second and third segments well supplied with setae; prominent spine at outer distal corner of the sympod.
Antennal scale: short, lanceolate, with the outer margin less convex than the inner; about four times as long as broad; distal suture present cutting off a very small segment; setae arming the margins unusually closely set and numerous.
Eyes: very large, set very close together and borne on small triangular stalks; extending forward beyond the distal end of the second segment of the antennular peduncle; flattened dorsoventrally. On the inner side of the small eyestalk there is a small hump which is not sufficiently pronounced to be regarded as an ocular papilla.
Thoracic limbs: first thoracic limb with the endopod very robust and densely furnished with setae which are armed with spinules on one side only; propodus twice as broad as long; long slender nail present. Second thoracic limb with endopod very massive and strong, the basal and the third segments each produced on the inner side and densely setose; merus somewhat slender; propodus densely armed with strong setae which are only barbed along one side. Remaining thoracic endopods moderately short, strongly built; fused carpopropodus divided secondarily into four segments of which the first is the longest; dactylus very small; nail long and very slender. Genital appendage on the eighth thoracic limb of the male short and barrel-shaped, armed along the anterior margin with a row of short plumose setae, apex armed with five or six curved bristles. Pleopods of the same type as Leptomysis. The exopod of the fourth pair has no modified setae on the second and third penultimate segments but bears a single very long modified seta at its tip.
Telson: in the form of a long and narrow trapeze; somewhat longer than the last abdominal somite; lateral margins straight, converging posteriorly, armed with about twenty strong, subequal spines; apex very narrow, either emarginate in the middle line or with a distinct unarmed notch; apex armed on each side with three spines of which the middle one is very long, the inner about one-third, and the outer about half, of the length of the middle one. A pair of plumose setae arise just anterior to the base of the notch on the ventral surface. The telson is hollowed dorsally and so has somewhat the shape of a narrow trowel.
Uropods: endopod armed with a short row of two to five spines which are set in a curve around the inner margin of the statocyst.

Colour
The body is clear and transparent, of a suffused red colour shading to purple especially on the ventral side of the abdomen. The marsupium in the female is of a deep red colour. Pigment of eye black with a brownish tinge.

Size
Length of the female, 25 mm; of the male, somewhat less.

Depth range
Hyperbenthic. The species is a bottom-living, deep water form confined to northern waters. It has not yet been recorded from the surface.

Remarks
The apex of the telson in this species shows considerable variation. In some individuals it is almost straight, showing only the faintest indication of an emargination in the middle line whilst in others there is a quite deep, smooth notch which is rounded at its proximal end. This variation is in no way connected with growth changes for straight or notched or intermediate forms may occur in animals of all sizes.

Distribution in the North Sea
Northern and north-eastern North Sea, W Norway, Skagerrak, Oslofjorden.

World distribution
E North Atlantic 44-70°N; shelf to slope.

[After Tattersall and Tattersall, 1951]

Mysideis insignis