Shell small, fusiform or widely fusiform. Protoconch cyrtoconoid with three or more evenly convex glossy whorls. Teleoconch with 4.5-7.0 evenly convex whorls. Sculpture on late teleoconch whorls dominated by spiral elements, some species may lack sculpture and single adapical spiral groove may be present. In contrast, earlier teleoconch whorls sculptured by coarse axial ribs. Siphonal canal rather long, straight, and tapering, or slightly recurved at its tip. Aperture rather wide, with evenly convex, sometimes slightly undulating, outer lip; inside of outer lip smooth or lirate within. Inner lip with three or four narrow columellar folds of equal or subequal strength. Head-foot massive, with light-brown or black bands on dorsal surface of foot. Eye tentacles moderately long, with large eyes situated at bases. Radula with rather broad tricuspidate rachidian and narrow unicuspidate sickle-shaped laterals. Gland of Leiblein large, glandular, brown. Males with seminal canal represented by open groove within mantle cavity, and closed along entire length of penis. Etymology: From the Latin costa (rib) and apex (top). The name Costapex refers to the characteristic sculpture of early teleoconch whorls.

Distribution and habitat
Central Indo-Pacific, from the South China Sea to New Caledonia, one species in the Caribbean, from Guadeloupe to Curacao, in the lower subtidal and bathyal depths; most species are sampled from sunken wood.
Remarks
Species delimitation based on the Cox 1 fragment resulted in 10 SSHs within the genus Costapex . Three of them were named with certainty based on the inclusion of the sequenced type specimens of C baldwinae (SSH 152), C. levis (SSH 23), and C sulcatus (SSH 68). Specimens in further three SSHs, 65, 66 and 211, were found to conchologically match the type specimens of, respectively, C martinorum (Cernohorsky, 1986), C.joliveti (Poppe & Tagaro, 2006), and C. ronnyi (Poppe, Tagaro, & Salisbury, 2009) comb. nov. and were identified accordingly. The remaining four SSHs do not match any described species, and are represented by sufficient material, so they are described as four new species: Costapex dolichosulcatus sp. nov. (SSH 185), C. ancistrolepis sp. nov. (SSH 67), C. mariavittoriae sp. nov. (SSH 46), and C pyramidalis sp. nov. (SSH 69). Despite a high degree of genetic differentiation that would warrant separate species status to the delimited SSHs,
identification of some of them based on morphology is challenging. Three (C. sulcatus, C. dolichosulcatus, and C ancistrolepis) form a complex of semi-cryptic species, where shell characters alone may not be sufficient to distinguish species. Among these three, C. dolichosulcatus is sister to C. ancistrolepis, but they are not closely related to C. sulcatus. We suspect that the unsequenced paratype of C. sulcatus (Fedosov et al. 2017: Fig 15 C) may actually be a C. dolichosulcatus, and therefore, the anatomical drawings and radula SEM images generated from that paratype represent C. dolichosulcatus. Similarly, C. pyramidalis is morphologically very close to C.joliveti, and even though a thorough comparison has revealed some subtle differences between them, their delimitation would not have been possible without prior DNA data.
The frequent erosion of shell surface and the prevalence of spiral sculpture in most species of Costapex may be associated with the characteristic habitat of Costapex, namely sunken wood.

Because of the lack of axial sculpture on late teleoconch whorls or prevailing spiral sculpture, the overall appearance of Costapex species recalls deepwater Mitridae, rather than costellariids. The species of Costapex can be readily distinguished from mitrids by the presence of strong axial ribs on early teleoconch whorls. In all species of the Costapex the columellar folds are thin, of equal or subequal strength; this allows their easy recognition among other deepwater costellariids, which usually have the adapicalmost columellar folds notably stronger. The fine columellar folds and often small shells of some Costapex species also render them extremely close to the volutomitrid genus Microvoluta. The two genera, however, can be distinguished by the details of columellar folds: these are always rounded in Costapex and sharp in Microvoluta. In addition, the presence of a translucent, bulbous protoconch readily reveals the species of Volutomitridae.
Six living species, which we assign to the newly established genus Costapex, can be separated into three morphologically distinct groups. The 'lesser' Costapex species, Costapex sulcatus, Costapex joliveti, and Costapex margaritatus, are characterized by small shells, which do not usually exceed 15 mm in height, and late teleoconch whorls dominated by spiral sculpture, whereas earlier whorls bear strong axial ribs. The 'larger' Costapex species, Costapex martinorum and Costapex exbodi are similar to the 'lesser' species in sculpture pattern, but are characterized by solid, notably larger shells that are intensely coloured brown. The third group, which can be referred to as 'smooth' Costapex is represented by the single species Costapex levis, which is distinctive because of its smooth late teleoconch whorls.

Species of Costapex are recorded from the tropical western Pacific: Philippines, Papua New Guinea (Bismarck and Solomon Seas), New Caledonia. At least two species of Costapex have been collected from sunken wood.