Carapace fairly elongate and tapering anteriorly (C. oblonga Habitus 1, C. oblonga Habitus 3). Postero-dorsal corner about 80°, the right carapace valve carries a small but distinct spine, left asymmetrical gland opens close to the corner but just anterior of the end of the hinge. The right asymmetrical gland opens at a small notch, but there are two forms of this species, form A in which the gland is situated at the postero-ventral corner, and form B in which the gland opens a little anteriorly to the corner. Apart from this small, but clear and consistent difference in the location of the gland, there seem to be no other differences either in the morphology or the ecology of the two forms. Three or four parallel striae occur along the ventral margin of the carapace, but there is no other sculpturing.
Female, frontal organ partially differentiated with stem a little longer than the first antenna, capitulum hirsute along ventral surface, ends in a sinuate point (C. oblonga 1). First antenna segmented, second podomere with a long dorsal seta extending to midpoint of capitulum, "e" seta with fringe of long hairs along its basal third, the fringe ends just proximally of the ends of the sensory setae.
Male, frontal organ stem just longer than first antenna, the capitulum thicker than the stem, down-turned, slightly curved and is bluntly pointed (C. oblonga 3). First antenna "a" seta as long as the limb, "c" seta short, "b", "d" and "e" setae are subequal, "e" seta armature about 25 pairs of basally pointing spines with three pairs of distally pointing smaller spines at the distal end (C. oblonga 4). Second antenna, right-hand hook appendage sometimes with inner barb, curved rather than angled at its base (C. oblonga 5), left-hand hook right-angled at base, distally straight.
The absence of sharp edged shoulder vaults immediately separates this species from its closest relative, C. spinifera (Conchoecia spinifera). Only form B has been recorded in British waters but, as both forms co-occur south of 35°N in the North Atlantic, form A may also be found.
Recorded from all oceans. 49°N-42°S in Atlantic. Generally shallow mesopelagic in its bathymetric range. 1 (R.R.S. Discovery Map).
None designated; status of original material uncertain.
Original description based on material collected from two stations in the North Atlantic at 37°45'N 13°38'W and 34°18'N 15°34'W. Initially confused with C. procera (Conchoecia procera).