Shell medium sized, thick, heavy, grey-blackish, low conical, non-umbilicated, iridescent aperture with aperture fold next to the columella, where a second fold occurs. Both folds inconspicuous.
Protoconch ocher to beige, 1.5 smooth whorls. Teleoconch about 3 whorls, gently increasing in diameter, no periostracum visible. Suture shallow and rather wide. Whorls sculptured by strong remarkably granulated spiral cords (the so called primary cords, marked PI, P2, P3.„- with P1 as the most adapical one). First teleoconch whorl sculptured by 5 strong high-elevated spiral cords of the same size. They are beset by flat, elongate to round oval granules. Cord diameter wide as space between them. PI and P2 of second teleoconch whorl coarser than the remaining 5. Granules on P1 and P2 more distinct than on other P1. P2 and P11are the most dominant cords on the last whorl. Their granules are merged towards the aperture and appear more lamellose than granular. There is a wide space between P10 and P11. Aperture more or less round, with a thick lip. Base convex and smooth.
Deeply retracted operculum thin, corneous, multispiral w uh narrow grow ing edge. Radula rhipidoglossate. Rachidian tooth wide, rhombic with a single w ide, triangular, slightly incurved dentate central cusp, with shallow tip. Lateral teeth 5 in number, unicuspid with serrated margins, rather short, with slender shaft and broad base. Inner marginal teeth slender, more than 30 in number, with very thin rounded tip, margins clearly serrated. Outer marginal teeth not clearly visible, but usually similar to the inner.
Ground body colour blackish fading to grey, foot underside light grey or white, cephalic tentacles black to dark grey, epipodial tentacles brighter. Head of animal with cylindrical snout, rather thick smooth eye stalks but eyes not visible in the critical point dried animal. Head without cephalic lappets. Roughly papillated cephalic tentacles. Neck lobes situated on head basal sides. Four pairs of epipodial tentacles both sides of foot . Anterior largest (on right side), all others of nearly equal size. Space between last two epipodial tentacles wider than others. Snout with rough end and small, smooth, spoonlike pseudoproboscis. Ctenidium not examined. Measurements (SW x SH in mm). 8.8 x 7.8 (holotype deposited in ZSM 20020494);

If we compare the radulae of Diloma samoaensis n. sp. and that of Diloma radula we found a close relationship between both species and the type species of Austrocochlea - a genus restricted to the Southeast Australian - Tasmanian region (Wilson, 1993; Hickman. 1998). Hickman & McLean, 1990 (fig. 60 E Austrocochlea constricta & fig. 60 F Diloma nigerrima) have shown details of radulae of both genera.

Although Sasaki (2000) placed the species described by Philippi, 1849 as Trochus (Diloma) radula in the generally accepted genus Diloma. an examination of material from Okinawa has shown that radula morphology is more closely related to the genus Austrocochlea than to Diloma (as illustrated by Hickman & McLean).
Further investigations of radulae of such species as Diloma piperimis (Philippi, 1849) and Diloma suavis (Philippi, 1849) may verify the propriety of their recent generic assignment.

D. samoaensis n. sp. differs from the closely related D. radula in several aspects;
- the general shell shape is much more globose in D. samoaensis than in the more depressed D. radula. (Figs 10-12)
- the sculpture of D. samoaensis is much stronger in the shoulder and basal spiral cords than in D. radula.
- the rachidian tooth of D. samoaensis has a more slender and narrower shaft at the upper end, and the blade is broader and lesser forward-directed than in D. radula.
- D. samoaensis, although reportedly living together with D. radula in the same geographical region, is restricted to isolated lava fields, where it lives very cryptically between the rocks and can only found active late at night, whereas D. radula lives under and on stones and is active during the entire day.

Type locality. Tutuila Island, American Samoa, between Pago Pago Harbor and Airport.
Habitat. Middle to upper littoral zone, extending to bottom of supralittoral, on black basalt boulders exposed to breaking waves at high tide.

Distribution. At present this species is restricted to only one spot at Tutuila Island, so we prefer not publish the exact type locality as over collecting might disturb or even destroy this population. The locality is, however, deposited with the type material and available to interested parties.