Diagnosis: Shell small, conical, peripherally carinate, umbilicate, nacreous under thin outer porcelaneous layer, white, polished, spire with flat to slightly concave sides, sculptured by widely spaced, sigmoid axial riblets. and fine spiral threads; peripheral carina strong, overlaid with several spiral threads, crenulated by terminations of axial riblets; base slightly convex, with strong, flattened spiral cords and / or narrow threads; umbilicus wide, deep, funnel-shaped, bounded In strong, tuberculate cord, aperture subrectangular; outer lip thin, with shallow posterior sinus occupying adapical half of whorl, claw-like near periphery; basal lip thin, with wide, shallow sinus in outer part; columella straight, slightly oblique, somewhat inflated medially, terminating in a rounded boss.

Remarks: Cossmann (1888) designated Basilissa superba Watson, 1879. as type-species of Basilissa Watson, 1879, although B. alta Watson, 1879, has been the species most frequently associated with that genus. Because that type-designation may not be changed, Hadroconus is erected for B. alta. B. sibogae Schepman. 1908, B. watsoni Dall. l927, and an undescribed western Atlantic species. Together, these four species form a morphologically cohesive species-group that may be distinguished from all other seguenziacean genera. Shells of these four species are small (< 10 mm), have spiral sculpture much weaker than axial riblets, or predominantly absent, the axial riblets are strongly sigmoid and opisthocline. and the umbilicus lacks a septum. Shells of B. superba are large (> 20 mm), have spiral sculpture subequal to axial riblets, axial riblets almost a simple prosocyrt are except for a weak opisthocyrt sinuation near the adapical suture, axials weakly prosocline, and umbilicus partially covered by a septum. In addition, shells of Hadroconus species usually have height: width (h:w) ratios less than 1.0, those of B. superba greater than 1.0; however, two specimens of H altus have h:w ratios of 103 and 1.07, and Okutani (1982) recorded two juvenile specimens of B. superba with h/w ratios of 0.96 and 0.98 (he did not indicate whether apical parts of these specimens are intact).
Shells of Hadroconus are most similar to those of Thelyssa Bayer. 1971, but differ in that the axial riblets are rather strongly sigmoid and opisthocline. and the umbilical walls lack callus and an umbilical septum. Hadroconus and Thelyssa are apparently sister taxa separated principally by the autapomorphic umbilical septum of Thelyssa.
Although I have not had an opportunity to examine a properly preserved specimen of any species of Hadroconus, dried specimens of both H. altus and H. watsoni were available for study and afforded the following observations of external anatomy. A well-developed epipodium bears four (or five) to six epipodial tentacles The mantle edge has two broad sinuses, corresponding to the basal and posterior labral sinuses of the shell; mid-dorsally is a prominent, seemingly papillate tentacle, to the left of which is a narrow, C-shaped sinus which corresponds to the peripheral angulation of the shell. The esophagus, seen in transparency after the animals were soaked in trisodium phosphate, extends posteriorly from the buccal area, sharply turns ventrally at the level of the last intestinal turn, and then runs posteriorly along the ventrolateral surface of the right intestinal tract. The intestine is similar to that of Seguenzia sp. cf. S. eritima Verrill. 1884 (Quinn, 1983b). but coils in the opposite direction; the shape of the fecal string is also similar to that of Seguenzia oval with a shallow medial groove on one side. The cephalic tentacles are long, papillate, appressed basally, and a long, slender penis arises just to the right of the right cephalic tentacle. On either side of the snout is a prominent, paddle-shaped oral lappet.