Waldo arthuri Valentich-Scott, Ó Foighil & Li 2013

Shell extremely thin, fragile, moderately inflated, translucent; equilateral to slightly longer posteriorly, anterior end slightly flared to gently sloping (Figure 1A-C); shell margins only weakly gaping if at all. Prodissoconch non-umbonate, D-shaped, with a greatly reduced PII comprised of a small number of faint commarginal striae bordering the metamorphic prodissoconch/dissoconch boundary (Figure 1D), prodissoconch length ranged from 338 to 357 µm (n=8) (Figure 1B). Dissoconch sculpture of commarginal striae, plus low broad irregular radial ribs; external sculpture variable, radial ribs absent to moderately strong, especially on anterior and posterior ends in some specimens. Beaks low, wide. Hinge plate extremely narrow, edentulous (Figures 1E, F). Length to 5 mm. Mantle large, reflected, covering approximately 80% of outer shell surface when fully extended, not covering umbones (Figure 1G); mantle can be completely retracted into the shell; reflected portion papillate (Figure 1H); fused posteroventrally; facultative exhalant siphon, trumpet-shaped, non-papillate; anterior end thin, non-papillate. Mantle tentacles long, extend well past shell margins (Figure 1G). Adult with projecting anterior pair, two laterally projecting pairs just posterior to anterior tentacles (one pair on each side); lateral tentacles not present on individuals less than 1 mm in length; ventral pair of tentacles just anterior of exhalant siphon (largest of all tentacles, in adults up to length of shell); single posterior tentacle projects dorsally to the exhalant opening. When animals are actively crawling, it appears that the tentacles might be used as levers to navigate between the urchin spines. Foot large, exceeds the length of the shell when fully extended, vermiform, without heel (Figure 1G); long ventral byssal groove extending to end of smooth foot tip. This species is an active crawler, and can also attach to the host by byssal threads. Ctenidia with one demibranch on each side, comprised of about 12-15 widely spaced filaments in larger specimens.

Canada, British Columbia, Barkley Sound, Sanford Island, 80 meters, and Imperial Eagle Channel in soft sediments; and United States, California, from Monterey Bay to La Jolla, from 113 to 444 meters [SBMNH]. Ten juvenile specimens from the intertidal zone of Smeaton Bay, Alaska (55.4°N, 130.6°W) [SBMNH 149330] are too small to be identified to species, but might also be Waldo arthuri.