(Figures 1–6)
Type material: Holotype, MZSP 77988 [Lt 1.16 mm; Wt 0.80 mm], off the Rio Grande do Norte, Brazil, D- 26, 06º 14 ’04’’S, 34 º 52 ’ 33 ’’W, 510 m, muddy bottom, 26.xi. 2001; 1 paratype, MZSP 77989, off Rio Grande do Norte, Brazil, D-23, 04º 51 ’00’’S, 35 º06’ 46 ’’W, 375 m, muddy bottom, 24.xi. 2001; 2 paratypes, MZSP 77990, off Rio Grande do Norte, Brazil, D-27, 06º 14 ’ 24 ’’S, 34 º 52 ’06’’W, 500 m, muddy bottom, 26.xi. 2001; 1 paratype, LMUFRPE 0 22, off Rio Grande do Norte, Brazil, D-27, 06º 14 ’ 24 ’’S, 34 º 52 ’06’’W, 500 m, muddy bottom, 26.xi. 2001.
Type locality: Off the state of Rio Grande do Norte (06º 14 ’04’’S, 34 º 52 ’ 33 ’’W), continental slope of northeast Brazil, at a depth of 510 meters.
Diagnosis: Shell low-conical, convex contour, body whorl increasing rapidly in diameter, compared to previous whorls. Penultimate whorl low in relation to diameter. Circular aperture, very expanded peristome, lower part of aperture being drawn out towards the length axis.
Description: Shell minute [Lt 1.12 to 1.18 mm], translucent or white, thin, low conical shape, holotype Lt/Wt 1.45. Protoconch globose 1 ½ whorls, smooth, nucleus inflated, slightly inclined (Fig. 6). Teleoconch beginning with emergence of strong orthocline threads, at times with aspect of axial cords, disappearing into subsequent whorls; having 2 ½ post-embryonic whorls, with rounded contours; body whorl increasing rapidly in diameter. Penultimate whorl very low in relation to diameter. Shell entirely polished, with irregular growth scars. Suture well-marked, no ornamentation on margin. Spiral striae microscopic, visible at magnification only in subsutural area (Fig. 2). Base conical, slightly convex. Aperture wide, circular, peristome strongly reflected, expanded forward, lower part of aperture being drawn out towards center of shell. Parietal region weakly reflected with no thickening. Umbilical narrow below reflection of inner lip. Inner lip expanded, umbilical notch narrow (Fig. 5).
Etymology: Macros, Gr. = large and Stomatos, Gr.= mouth, in reference to the broad shape of the aperture.
Geographic and bathymetric range: Off the state of Rio Grande do Norte (04º 51 ’00’’ – 06º 14 ’04’’S; 34 º 52 ’ 33 ’’ – 35 º06’ 46 ’’W), northeast Brazil, at 375 to 510 m depth.
Remarks: The small number of whorls exhibited by the teleoconch of Aclis macrostoma makes any comparison with similar species from the Atlantic difficult, as this species presents a very distinct pattern. The soft parts are unknown, but the holotype demonstrated that shell was fresh, due to its vitreous aspect (Bouchet & Warén 1986: 302). The strongly expanded outer lip, deep umbilicus and circular aperture are not characteristics commonly found in congeners from the North Atlantic, but the group that includes Aclis benedicti (Bartsch, 1947) illustrated in Bartsch (1947: pl. 5, fig. 5) and Aclis attenuans Jeffreys, 1883 illustrated in Bouchet and Warén (1986: figs. 730–732) also presents this dilatation of the peristome. Apparently, the rate of expansion in A. macrostoma is one of the largest ever seen, producing a strongly conical profile that is not common among other species of Aclididae. In Aclis attenuans, the convexity of the smooth whorls, body whorl and chink on the base are similar to A. macrostoma.
The planispiral Discaclis canariensis can be matched with Aclis macrostoma regarding size and the similar number of whorls on the protoconch and teleoconch, as well as in the shape of the aperture with a nonthickened outer lip. The former species is broader and has a channeled suture and strong deep basal keel, which are sufficient characters for separating it from A. macrostoma.
Aclis gulsonae (Clark, 1850) is similar to Aclis macrostoma regarding the prominence of the outer lip and the nearly straight inner lip, as well as the presence of fine, irregular growth lines. As observed by Jeffreys (1869: 106), the former species is much more slender and truncated (6 to 7 whorls), more cylindrical rather than conical, with a deep suture.
Aclis macrostoma and Aclis sarissa present few relevant characters in common. Among these are rounded, glassy and smooth whorls, except for faint growth lines. The last whorl is disproportionately large, well rounded, tumid, and with the suture linear and slightly oblique. In Aclis sarissa, the tip of the apex is prominent. The aperture is oval, with the outer lip slightly narrow, mainly at the union with the body whorl, and the inner lip is thin and sharp. Furthermore, the umbilicus is referred to as funnel-shaped (Watson 1886).