Diagnosis
Shell fusiform to broadly fusiform, smooth, with orthoconoid spire and rather long siphonal canal. Protoconch pointed, dark brown, with large nucleus, consisting of three or more smooth whorls. Suture distinct, impressed, whorls slightly convex in outline. Early spire whorls sculptured with widely spaced coarse axial ribs, intersected apically by single groove. Axial ribs disappear on fourth teleoconch whorl, subsequent whorls lacking sculpture, sometimes retaining adapical groove only. Microsculpture of very fine collabral growth lines. Last adult whorl evenly convex with elongate shell base and rather long, tapering, widely open siphonal canal. Aperture elongate with thin, evenly convex outer lip bearing faint lirae on its inner surface. Inner lip with four strong columellar folds, adapicalmost fold slightly exceeding others in strength. Shell colour whitish to pale or tan.

Description
Shell small, elongate-fusiform (w/h 0.31-0.39; a/h 0.45-0.51), with rather tall spire and long, tapering siphonal canal. Protoconch pointed, dark brown, with large nucleus, consisting of three or more smooth whorls. Teleoconch of about seven slightly convex whorls, first three with strong orthocline axial ribs (14 on third teleoconch whorl), giving an indistinctly shouldered profile. Axial ribs intersected by strong spiral groove forming subsutural 'caps', noticeable on early teleoconch whorls. Axial ribs becoming broader and indistinct on fourth teleoconch whorl, and vanishing entirely on succeeding whorls. Last three whorls smooth, sculptured only by very fine growth lines, slightly convex in outline. Suture distinct, very slightly impressed. Last adult whorl evenly convex, forming concave waist with five strong spiral grooves at its transition to siphonal canal. Siphonal canal long and slender, tapering, with rather deep and narrow opening. Aperture elongate, outer lip thin, evenly convex, with very faint lirae on its inner surface. Inner lip with four strong but rather narrow columellar folds, adapicalmost strongest, then gradually reducing in size. Colour uniformly white, with interspaces between spiral ribs on early teleoconch whorls retaining light-brown periostracum (or possibly stained so by mud).
Radula about 0.4 mm long and 0.1 mm wide, consisting of about 60 rows. Rachidian with rather wide bow-shaped base and three moderately long pointed cusps; anterior part of rachidian base with sharp, blade-like edge (marked with white arrows). Laterals unicuspidate, sickle-shaped with slightly widened base.
Morphology of the body: The holotype, a male, has well-developed eyes situated at the base of the tentacles. Penis rather small, seminal groove closed. Etymology: The name of this species emphasizes the lack of the sculpture on late teleoconch whorls (Latin levis = smooth, plain, free from coarse hair), which distinguishes Costapex levis from congeners.

The material examined shows significant variation in shell proportions, from the fusiform holotype to elongate with high spire and relatively shorter aperture. At present, we are inclined to attribute this variation to bathymetry, as MNHN IM-2013-40629 and an even more elongate specimen from the Solomon Islands were sampled from depths greater (700-740 and 1077-1135 m, respectively) than the others (398-619 m); however, it cannot be excluded that we are dealing with two separate species that do not overlap in bathymetric distribution.
Because of their smooth adult teleoconch whorls, both the typical and elongate forms stand apart from other costellariids. At the same time, the coarse axial ribs on early teleoconch whorls readily distinguish Costapex levis from species of mitrids. In overall shell shape, Costapex levis shows a certain resemblance to the boreal Volutomitra groenlandica (Moller, 1842) (Volutomitridae); however, it can be distinguished by the presence of axial elements on early spire whorls and by the comparative strength of columellar folds. In Costapex levis the adapicalmost fold is the strongest, whereas in Volutomitra groenlandica it is notably fainter than the succeeding ones.

Distribution and habitat
Philippines, Papua New Guinea (Solomon Sea), Solomon Islands, depth range 280-1077 m. One of the specimens studied was collected from sunken wood.