Protoconch. Moderately tall, of about 3.5 ad-pressed whorls (exact count could not be made from available specimen), and with well-developed outward-flaring lip and sinusigeral notch. Angular, ridged shoulder on last 1.5 whorls. Series of vertical plicae on area close to end of last whorl. Subsutural plicae on last 2.5 whorls. Increasingly well developed ridged shoulder with subsutural plicae. Last whorl with small pustules and with vertical plicae on last quarter whorl.
Shell Morphology. Shell moderately high-spired (apical half-angle of spire 30° to 36°), sutures between teleoconch whorls distinct; umbilical slit absent; siphonal canal short, broad, forming upturned notch at basal end, and set off from remainder of last whorl by prominent fasciole. Teleoconch whorls sculptured by major nodose spiral cords and by numerous exceedingly fine spiral threads; on last whorl up to five primary cords, one at the suture, two central ones with prominent low rounded nodes, and two subordinate basal cords of which one (the most basal cord) is often missing. Outer lip crenate at edge, bearing short, sharp labral spine at position basal to abapical (fifth) primary spiral cord and situated at the end of a shallow external groove. Inner edge of outer lip with four weakly elongate denticles situated close to the edge of the lip; inner surface of outer lip otherwise smooth, not lirate. Adapical end of aperture constructed by prominent parietal rib; no anal slit. Columella straight, with more or less distinct central fold extending into the aperture and one or more weak basal riblets that are confined to the ventral surface of the inner lip.
Shell Microstructure. Inner layer of aragonite with crystals oriented in a 45-degree angle to the growing edge; middle layer of aragonite with crystals oriented perpendicular to growing edge; outer layer of aragonite with crystal planes oriented parallel to growing edge. Inner layer absent in some specimens.
Operculum. D-shaped, but upper end more rounded, with lateral nucleus in center right. Free surface with bracket-shaped growth lines; attached surface with 3-4 bracket-shaped growth lines and with callused, glazed rim (about 30-35% of opercular width) on left.
Anatomy. The anatomy of Acanthais brevidentata is typically rapanine, and schematic drawings of anatomical features, as well as detailed explanations of terminology used, can be found in previous papers by Kool (1989, 1993a, b). It should be noted that coloration given for the anatomical features described below is found in the living animals and may have faded or disappeared following preservation in alcohol.
Head and foot. Foot flecked with grey, black, and orange; edge primarily orange with minute while specks; sole opaque grey. Cephalic tentacles elongate, thin, dark brown with grey tips. Incurrent siphon with white and orange specks proximally, flecked with black distally. Accessory boring organ elongate, well developed; in females dorsal to short ventral pedal gland.
Mantle cavity. Osphradium about two-thirds of ctenidial length, equal in width to ctenidium. Right pectin wider than left. Each lamella (12-15/mm) attached to mantle roof along one-half its length. Anteriormost portion of ctenidium curving around anterior osphradium. Ctenidial lamellae (14-17/mm) with strongly convex lateral edges, more wide than high anteriorly, more high than wide posteriorly. Supporting rods extending beyond lateral edge of each lamella, forming small papilla.
Radula. Ribbon length 30-35% of shell height. Central cusp of rachidian wide, flame-shaped; inner lateral denticle low on base of lateral cusp; outer edge of lateral cusp with one denticle (in some specimens only a remnant); marginal area with two marginal denticles and well-developed marginal denticle. Lateral teeth nearly equal to width of rachidian tooth.

Comparative remarks: Protoconch. The protoconch is typically rapanine; it is multispiral with 3+ whorls (exact figure cannot be given due to imperfect preservation of protoconch specimen) as opposed to the typical paucispiral protoconch in ocenebrines of 1.5 whorls; and the outward-flaring lip and sinusigeral notch are indicative of a planktotrophic larval stage, found in all rapanines and absent in ocenebrines. The Acanthais protoconch is similar to that of Stramomta in both overall shape and sculptural pattern.
Shell Morphology. Acanthais bears a strong superficial resemblance to three ocenebrine genera, Acanthina Fischer von Waldheim, 1807 (type species: A. monodon (Pallas, 1774), southern South America); Acanthinucella Cooke, 1918 (type species: A. punctulata (Sowerby, 1835), California); and Spinucella Vermeij, 1993 (type species: S. tetragona (Sowerby, 1825), Pliocene, North Sea Basin). All four genera possess a labral spine and strong spiral cords. Acanthais differs from members of the Ocenebrinae by columellar, apertural, and sculptural characters.
Like many Rapaninae, Acanthais has sculpture on the columella. In Acanthaisy there is a central fold. This fold is much weaker than the central fold of Cymia Morch, 1860 (type species: C. tecta (Wood, 1828), tropical eastern Pacific). The latter genus differs widely from Acanthais in its lirate aperture, fine spiral ribs, adapical apertura! notch, and strong peripheral tubercles. Ocenebrines have a smooth columella. Some rapinines also have a smooth columella. These include Concholepas Lamarck, 1801 (type species: C. concholepas (Bruguiere, 1789), western South America); Rapana Schumacher, 1817 (type species: R. bezoar (Linnaeus, 1758), Indo-Pacific); Ecphora Conrad, 1843 (type species: E. quadricostata (Say, 1824), Pliocene of Virginia); and limpetlike morphs of Plicopurpura Cossmann, 1903 (type species: P. columellaris (Lamarck, 1816), tropical eastern Pacific). These taxa are low-spired to limpetlike, and have a sculptural pattern different from that of Acanthais and related genera.
In Acanthais. there is a parietal rib that terminates as a distinct knob at the apical end of the aperture. This feature is absent in most Ocenebrinae, including Acanthina and Spinucella. Some species of Acanthinucella and many populations of Nucella emarginata (Deshayes, 1839) from California possess a parietal tubercle, but this feature is discrete and is not a spiral parietal rib as it is in Acanthais and most other Rapaninae. A similar parietal rib occurs in members of Muricinae.
The five-rib pattern of major spiral cords on the body-whorl sets Acanthais apart from Ocenebrinae but links it to many members of the Rapaninae. The adapical cord, or subsutural ridge, lies just below the suture and periodically forms apically directed extensions that produce weak knobs. At the outer lip, the most recently formed of these extensions produces a gutterlike extension with the opposing terminus of the parietal rib. The subsutural cord and corresponding posterior apertural gutter are found in nearly all Rapaninae but are lacking in Acanthina, Acanthinucella, Spinucella, and other Ocenebrinae. Most ocenebrines have seven or more primary spiral cords. Members of the Acanthinucella lugubris (Sowerby, 1821) group from northwest Mexico (see Wu, 1985) have only four primary spiral cords, the two apical ones bearing nodes and the two basal cords being broadly rounded and set close together. Recent specimens of the Nucella emarginata species complex have five major cords, but there is no subsutural cord as in Rapaninae.
Among rapanine genera, Acanthais bears a strong superficial resemblance to Stramonita Schumacher, 1817 (type species: S. haemastoma (Linnaeus, 1767), Mediterranean and West Africa). In both genera, cords 2 and 3 on the body whorl (counting from the suture) are adorned with a row of equally prominent nodes. The primary cords of Stramonita, however, are narrow and angular, whereas those of Acanthais are low, broad, and round-topped. Moreover, the two genera differ in secondary spiral sculpture and in the way that the primary cords are reduced in many populations. In Acanthais, cord 5 (the most basal one) is rarely expressed, so that the primary sculpture appears to consist of three closely spaced cords on the central portion of the body whorl and a very weak noded subsutural cord. The entire shell surface is overlain by extremely fine spiral threads. In populations of Stramonita in which the primary spiral cords are reduced, the primary cords are of the same size as, and are difficult to distinguish from, the relatively coarse secondary cords or riblets. This is especially so in western Atlantic populations of S. haemastoma (belonging to the subspecies floridana Conrad, 1837, and canaliculata Gray, 1839), in the eastern Pacific S. biserialis (Blainville, 1832; Figure 1D-F), and in the Peruvian S. delessertiana (Orbigny, 1941). These typical species of Stramonita also differ from Acanthais by the presence of riblets (lirae) on the inner side of the outer lip. Lirae are absent in Acanthais. Moreover, typical species of Stramonita lack the central columellar fold of Acanthais, and also lack a labral spine.
The tropical eastern Pacific genus Neorapana Cooke, 1918 (type species: N. muricata (Broderip, 1832)) resembles Acanthais in possessing a short labral spine in a position just basal to cord 5. In shell characters, Neorapana differs from Acanthais by lacking the central columellar fold, by having the inner side of the outer lip Urate rather than smooth, and by possessing an umbilical slit.
Several Indo-West-Pacific rapanines are also superficially similar to Acanthais. Thais aculeata (Deshayes & Milne Edwards, 1844) is the type of Thalessa H. & A. Adams, 1853, but was assigned to Stramonita by Fujioka (1985) on the basis of radular characters and to Mancinella Link, 1807 (type species: M. alouina (Roding, 1798), tropical Indo-Pacific) by Trondle & Houart (1992). This species and its relatives have a weak to obsolete central col¬umellar fold and, like Acanthais, a nonlirate outer lip bearing denticles on its inner side. However, whereas it is the fifth (most basal) primary spiral cord that is often missing in Acanthais, the fourth cord is usually weakest in the T aculeata group. Strong erect spines adorn all five cords or all but the fourth cord in Thalessa. The coarse secondary spiral sculpture of Thalessa resembles that of Stramonita rather than the fine threads of Mancinella or Acanthais. It may be that Thalessa is close to Stramonita, but further anatomical study is needed to clarify the placement of this group.
Finally, Acanthais shares many features with Thais Roding, 1798 (type species: T. nodosa (Linnaeus, 1758), tropical West Africa). Species of Thais resemble Acanthais in having exceedingly fine secondary spiral sculpture and in lacking lirae on the inner side of the outer lip, but they lack a central columellar fold and a labral spine. In T. nodosa, all five primary spiral cords are usually well developed and bear nodes or tubercles; but in the closely similar T. meretricula Roding, 1798, from the central Atlantic islands, the shell surface is covered by very fine spiral sculpture and lacks distinct primary cords.