Diagnosis: Overall shell profile markedly rounded, roughly as wide as tall; spire slightly step-like, region below suture markedly concave. Body whorl broad. Teleoconch sculpture delicate and narrowly spaced. Spiral cords faint, giving whorls a more convex profile.
Description: Shell small (height 2mm), rounded, conical to turbinate (roughly as wide as tall), relatively thick-walled; spire short, height 1/2 of shell height, spire angle 80°; color nacreous glossy white. Protoconch (-1 whorl, 0.28 mm) bulbous, with pitted, irregular surface, not too prominent; transition visible as thin orthocline line. Teleoconch of circa 4 whorls, with well-marked, but not particularly deep suture. Outline concave between cords, but with an overall convex profile; region slightly below suture markedly concave; sculpture consisting of delicate but well-marked spiral cordlets crossed by axial threads, forming a delicate reticulate pattern; disposition of axial threads alternating between prosocline, orthocline and opisthocline between cords along the shell's longitudinal axis, constituting an overall continuous sigmoid pattern; spiral sculpture becoming gradually weaker toward basal region of last whorl. Body whorl base sculptured by 10-12 spiral cords, with axial threads also present between cords. Umbilical region smooth, edge rounded but thick and well-marked. Aperture quadrangular, height 1/2 shell height, with three labral sinuses: a deep, large (1/5 of whorl height) subsutural sinus; a small (1/6 of aperture height), very shallow sinus in medial region of outer lip; and a relatively large (nearly same size as subsutural) sinus on basal region of aperture, congruent with umbilical carina. Umbilicus narrow (diameter 1/7 of shell diameter), deep. Measurements (in mm): Holotype: 4,5 whorls, H = 1.78; D = 1.75; h = 0.71;d = 0.64.

The present specimens fit well in the genus Halystina, described by Marshall (1991) and corresponding to the Seguenzia Group III sensu Quinn (1983b). The genus is known almost exclusively from the South Pacific, with a single species, H simplex (Barnard, 1963), occurring off Cape Point, South Africa (Marshall 1991; Poppe et al. 2006). The genus is diagnosed by its small size, overall rounded shell profile, with flat-sided whorls and finely granulate protoconch (Marshall 1991) and is probably related to the very similar genus Halystes Marshall, which differs almost exclusively by its much larger size (Marshall 1988).
The present specimens are reminiscent of most of its congeners, but can be diagnosed by its more rounded overall profile, with a broader body whorl and a more step-like spire, a distinct concave region below the suture, fainter spiral cords (which gives the whorls a more convex profile) and its more delicate and narrowly spaced teleoconch sculpture. The present record greatly expands the distribution of the genus Halystina to western South Atlantic waters.
Halystina umberlee is most similar to H vaubani Marshall, 1991, but can be distinguished by its broader body whorl, more delicate teleoconch sculpture and fainter spiral cords. Halystina umberlee can be distinguished from H siberutensis (Thiele, 1925), H caledonica Marshall, 1991 and H globulus Poppe, Tagaro & Dekker, 2006 by its more rounded profile, more convex whorls and more step-like spire. Finally, it can be easily differentiated from H simplex and H cahnata Marshall, 1991, which have more angular shell profiles similar to specimens of Seguenzia Jeffreys, 1876.

Type locality: Brazil: Espirito Santo: off Sao Mateus, continental slope of Abrolhos, 19°00'S 37°48'08"W, 950-1050 m depth (MD55, sta. DC72, 27/v/1987).
Distribution: Off southeastern Brazil, from Espirito Santo to Rio de Janeiro, at 950-1550 m depths.

Etymology: The specific epithet is in apposition named after Umberlee, a fictional goddess of the deep sea from the Faerunian pantheon of the Forgotten Realms campaign setting of the Dungeons & Dragons role-playing game.