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(Brady, 1902)

Diagnosis
Carapace of both sexes elongate, height less than half the length (A. cucullata Habitus 1, A. cucullata Habitus 3). Female carapace tapers anteriorly with maximum height at two-thirds the length (A. cucullata Habitus 1). Dorsal and ventral carapace margins almost parallel in male (A. cucullata Habitus 3). The unequal rostra are straight and produced anteriorly, left larger, nearly quarter the carapace length. Both valves with pointed processes on posterior margin. Process on left valve located just below postero-dorsal corner; process on right valve longer and positioned slightly more ventrally. Clear and distinctive sculpturing of parallel striae run in differing directions on dorsal third of carapace to the ventral two-thirds. Striae end in spines along ventral margin from the postero-dorsal spine to about where the ventral edge reaches the point of maximum height. Frontal organ long and thin in both sexes; it is half as long again as the limb of the first antenna (A. cucullata 1, A. cucullata 3).

Remarks
Poulsen, 1969a noted the existence of two size forms; this is confirmed by Discovery Collections material. These different size forms are probably distinct species. Smaller species (A. cucullata Habitus 5) are more abundant and sometimes sub-dominant at deep mesopelagic depths at lower latitudes. Larger sensu stricto form (Brady, 1902 described its length as being 2.5mm) is the more common in UK waters. Sampling at abyssopelagic depths reveals taxonomic uncertainties; at least two further species have been taken. one in UK waters (see A. aff. cucullata (Archiconchoecia aff. cucullata)). The strikingly different carapace morphology and mandibular teeth of these species compared with other Archiconchoecia species may segregate them generically.

Distribution
Occurs in all oceans between 60°N-50°S. 1, 2, 3 (R.R.S. Discovery Map). Where the two forms co-occur, the smaller species has the shallower bathymetric range. In UK waters larger form reasonably abundant at 600-1500m.

Type specimens
None designated; status of original material uncertain.

Type locality
Original material from three stations close to 52°N 15°W.

Archiconchoecia cucullata