Type locality: Bohol/Sulu seas, North of Dipolog, Mindanao, the Philippines. Diagnosis (DNA) [2 spms]: 'C in site 238, fG' in site 277, fCf in site 295, 'A1 in site 421.


Description
Shell small, fusiform (W/H 0.38, A/H 0.47), with high last adult whorl, and rather long, straight siphonal canal. Protoconch missing; teleoconch of ca 4,75 whorls, suture adpressed. Early teleoconch whorls distinctly shouldered, with well-developed subsutural band, separated from lower portion of whorl by deep groove; periphery of whorl weakly convex. Sculpture of orthocline ribs, very strong on periphery of early whorls, becoming notably weaker on later whorl, vanishing completely on last 1/3 of last whorl. Penultimate whorl with 15 axial ribs. Ribs overridden by low and broad spiral cords, rounded on early whorls, and flattened on last and penultimate whorls, adapical one corresponding to subsutural band; periphery with three subequal cords separated by narrow and shallower grooves, more pronounced in interspaces between ribs. Last whorl with gently convex profile above insertion of outer lip, marking transition to tall, evenly concave shell base gradually extending to siphonal canal. Sculpture of one further strong cord below subsutural band, followed by three broad flattened cords on whorl periphery, and 10 cords on shell base and siphonal canal.
Aperture elongate, lacking both a siphonal notch and an apical callous pad; outer lip evenly convex in its adapical 2/3, straight below; inside with 6 widely set lirae. Inner lip with well-developed white callus; columella with four fine, rather low folds. Shell surface chalky under thin periostracum, color light brown to beige; inside of aperture white. Etymology: The species epithet emphasizes the close similarity of the new species to the type species of Costapex, C. sulcatus, while referring to its somewhat more elongate shell.

Distribution: Currently known only from the Philippines.

Remarks
Costapex dolichosulcatus sp. nov. is very similar in shell morphology to C. sulcatus, and C ancistrolepis sp. nov., but can be differentiated from both by its slightly more elongate shell, weaker spiral grooves, and the columella bearing four folds, versus five in C sulcatus and C ancistrolepis. However, the available material is not sufficient for documenting intraspecific variation in either C. sulcatus, or C. dolichosulcatus, so differentiating the two species based on the cited shell characters may be problematic.