Holcostoma piliger (Philippi 1849)

Holcostoma piliger (Philippi, 1848)

Planaxis piliger Philippi, 1848:164.—Cossmann, 1906:196.

Planaxis (Holcostoma) piliger (Philippi).—Sowerby, 1877, pl. 5: fig. 38.—Thiele, 1929:204.—Wenz, 1940:722.

SHELL MORPHOLOGY (Figure 23).—Shell small, thin, ovoid with low spire, protruding nipple-like from an extremely large body whorl. Shell sculptured with deep spiral grooves on early whorls, weak, spiral grooves on penultimate and body whorls, with grooves becoming deeply channeled at siphonal constriction. Suture deeply impressed. Aperture large, about three-fourths of the shell length. Pseudoumbilicus present in mature specimens. Columella concave, broad at anterior siphon, with deep narrow anal canal. Outer lip smooth, expanded, slightly denticulate within and sinuous at upper end, when seen in side view. Anterior siphonal canal short, broad, distinct, with slight siphonal fasciole. Periostracum thin, brownish, markedly hispid, with individual hairs in regularly spaced spiral pattern. Shell color brownish with lighter yellow-white broad spiral bands, especially on body whorl. The lenticular operculum (Figure 24D) has a subterminal nucleus, is multispiral and nearly transparent.

ANATOMY.—Based on head-foot of one dried specimen reconstituted with trisodium-phosphate. The head has a narrow snout and small buccal mass. The long cephalic tentacles are tapered, each bearing a small black eye at the outer base. The edge of the mantle is slightly fringed but nearly smooth. The ctenidium is black and comprised of long triangular filaments.

Radula (Figure 24A–C). The radula is very small, about one-fifth the shell length. Rachidian tooth (Figure 24B) crescent-shaped, longer than wide, with a concave cutting edge comprising a single large serrated cusp. Rachidian formula . Basal plate with short central basal projection, long, narrow, lateral projections and two lateral basal denticles. Lateral tooth (Figure 24A,B) rectangular with a long narrow lateral projection that is attached to the basal membrane. Cutting edge with long, central cusp flanked on each side with 2 or 3 smaller denticles. Outer side of tooth with slight flange. Basal plate with central supporting column that extends slightly at the base. Marginal teeth (Figure 24C) with long basal shafts, curved and wider at tips and with many fine serrations. Shafts of marginals appear to be fused midway beneath the curled tips.