Figures 36 A–D

Type Material. Holotype: LACM 3163, 1 single LV 16.5 mm, 48 ribs. Paratype 1: LACM 3164, 1 single LV ca. 13 mm, 43 ribs, from the type locality. Paratypes 2–9: LACM 3165, 8 fragments, partly colored, 7.1–20.2 mm, dredged at 100 m, sand and rubble off Orongo, Easter Island, 27°11’58” S, 109°29’28“ W.

Type Locality. Dredged at 30–50 m in fine sand, off the western coastline near Tahai, Easter Island, 27°07’20” S, 109°26’30” W.

Description. A rather fragile and thin-shelled Acrosterigma species with a discriminating sculpture of 6–8 triangular ribs on the medio-posterior quarter of the shell. The largest complete valve, selected as holotype, measures 16.5 mm, but paratype fragments indicate a much larger size exceeding 30 mm; ovoid and nearly equilateral in shape, moderately higher than long, without a posterior truncation; umbones pointed, orthogyrate. Prodissoconch unknown, worn. The thin adult shells of the studied material are often fragmented. Complete valves are moderately inflated. Whitish in color, with irregular yellow-rose blotches; these are correspondingly found on the glossy white interior. Lunule virtually absent, ribbing almost reaching the margin; periostracum not observed. Sculpture of regular, rather broad ribs anteriorly and centrally. This regular sculpture changes into 6–8 acutely triangular ribs in the medio-posterior quarter, with the strongest sculpture expressed ventrally; the posteriormost 8–9 weakly but regularly spined ribs are again rather regular and broad. The sculpture on the anteriormost ribs is scaly; these scales are nearly commarginally placed and fade towards the central portion of the valve. Interstices are narrow, much narrower than the ribs, rounded and smooth. Rib count in the holotype: 48 (16.5 mm), and in the paratype 1: 43 ribs (13 mm). Hinge line weak and thin, asymmetrical with a closer posterior and a remote and stronger anterior lateral, ligament on a small, quadrangular nymph. Pallial line continuous; scars small, ovate, starting near the end of the laterals and extending ventrally.

Comparative diagnosis. The late Jacques Vidal considered this species to be in the genus Acrosterigma, and likely new to science. J.J. ter Poorten, (pers. comm., 2010) studied the material, comparing it with other described species and considered it a new Acrosterigma species as well. He noted that A. triangulare falls into Vidal’s (1999: 285) species-group of A. variegatum (G.B. Sowerby II, 1840), and is related to A. discus Vidal, 1999, and A. mauritianum (Deshayes 1854).

Acrosterigma discus is mainly an Indian Ocean species extending to the Philippines, but is not known from Australia or other tropical Pacific Islands. Its shape is more circular and the rib count appears to be from 53–63, with a mean of 56.3, which are more pronounced than in the EI species. The internal stains in the new species are yellowish-rose, instead of purplish found in A. discus. Finally, the rib sculpture in A. triangulare is characteristically acutely triangular in the medio-posterior quarter, whereas in A. discus this trait is less pronounced and the ribs are more rounded.

Acrosterigma mauritianum is currently only known from a very restricted area within the Western Indian Ocean, Mauritius (type locality) through Seychelles to Sri Lanka. Here, the rib count (41–47 and a mean of 44.1) as well as the coloring is comparable. However, the shape of A. mauritianum is distinctly angular posteriorly and straight ventrally. The ribs in the medio-posterior quarter of the valves are strongly broader and less acutely triangular than in the new species.

Acrosterigma variegatum, which ranges from Queensland, Australia to Melanesia, and as far north as Japan, is easily distinguished from A. triangulare. Similar sized specimens are very distinct by having a more quadrangular shape, fewer number of stronger ribs and more vivid colors, externally and internally. The hinge line is stronger and the anterior lateral tooth is closer to the cardinals.

Remarks. Here, the quality and number of specimens available is at the limit for a new description. Nonetheless it was ventured, as the two most eminent experts on cardiids independently came to the same conclusion. Furthermore, there is nothing similar known from the Hawaiian Islands. And in French Polynesia only the generically distinct Vasticardium mendanaense (G.B. Sowerby III, 1897), is readily found.

Distribution. Acrosterigma triangulare is occasionally found sublittoral, from ca. 30–100 m, and only known from a few locations around Easter Island— E1.

Etymology. The name refers to the characteristic rib sculpture on the medio-posterior quarter.