194 THE VOYAGE OF H.M.S. CHALLENGER. 



external appearance and the structural details, is very striking. But, as the frontal 

 plate exhibits in both a totally different form, and the eyes, moreover, in the present 

 form have a very light pigment, whereas it is dark in the Mediterranean form, I j^refer 

 nevertheless to regard both as distinct, though very closely allied species. 



Description. — The length of the body of the adult female is about 9 mm., that of the 

 male somewhat less. 



The form of the body (see PI. XXXIV. figs. 4-6) is remarkably short and thickset, 

 with the anterior division rather dilated, the posterior much more slender and cylindrical 

 in form. 



The carapace is comparatively very large, covering, besides the whole of the trunk, 

 also the anterior part of the first caudal segment. Its posterior margin, contrary to what 

 generally occurs in the Mysidse, is quite straight, without exhibiting the slightest trace 

 of the usual emargination in the middle ; and the lateral parts or wings are very deep 

 posteriorly, with their edges forming a strong curve. Anteriorly the carapace tapers 

 somewhat, and exhibits here the usual cervical sulcus, which is, however, not particularly 

 deep. The frontal margin juts forth in the middle as a rather broad, horizontally extend- 

 ing plate, abruptly truncate at the tip (fig. 7), not, as in the Mediterranean species, 

 acutely pointed. The antero-lateral corners of the carapace are but slightly produced 

 and almost right angled. 



The first caudal segment exhibits in the female (see figs. 4, 5), on either side, a narrow 

 plate, arching over the corresponding limbs, and apparently representing a kind of 

 epiniera. These plates are, however, wanting in the male (fig. 6). The other segments 

 are simple cylindrical, and the last of them somewhat longer than the rest. 



The eyes are comparatively short and thick, claviform, with the cornea slightly 

 expanded, and occupying about the outer half of the eye. The ocular pigment in all the 

 Challenger specimens, as also in those examined by Kroyer, is very light yellowish, 

 whereas in the Mediterranean form it is of the usual dark hue. 



The antennular peduncle (fig. 8) is rather thick and somewhat dilated toward the 

 end, with the second joint exhibiting a rather peculiar, as it were, cup-shaped form, being 

 apparently hollowed out anteriorly to receive the base of the terminal joint. The latter 

 is about as long as the basal, and exhibits in the male on the lower face a very short 

 knob-shaped lobe, clothed with a dense bunch of delicate hairs. Of the flagella, the 

 outer, as usual, is the longer, and exhibits at the base a short lamellar expansion bearing 

 a dense row of curved sensory bristles. 



The basal part of the antennae (see fig. 9) is rather thick and massive, and armed 

 interiorly, at the base of the flagellum, with a strong denticulate spine. The scale, on 

 the other hand, is exceedingly small, so as scarcely to be visible when the animal is 

 viewed from above (fig. 5). It exhibits a somewhat rhomboidal form, the apex being 

 obliquely truncate, with the inner corner slightly projecting and rounded off", the outer 



