Caecum magellanicum

Caecum magellanicum (di Geronimo, Privitera & Valdovinos, 1995)

Figures 96 A–E, 97A–F, 98A–B, 126F

Fartulum magellanicum di Geronimo et al. 1995: 114, figs. 1–5, holotype IPOP M1.8.8.95, Cabo Deseado, Strait of Magellan, Chile.

Fartulum magellanicum — Linse 2002: 85, pl. VII, figs. 54–56; Mattano & Zelaya 2013: 170.

Material examined. In addition to the type material the following lots were examined. NHMLAC: LACM 1973 - 75.4, Westhoff Island, Los Lagos Prov., Chile, 23 m, 8 sh; LACM 1973 - 74.5, Darwin Channel, Aysen, Prov., Chile, 8 m, 5 sh. FS: Stanley, near Yorke Point, East Falkland, Falkland Islands, 9 m.

Original diagnosis. “Teleoconch cylindrical, moderately curved, vitreous or lightly opaque in less fresh shells. Under the optical microscope, surface seems entirely smooth, with less distinct annular growth striae. Using the SEM these striae are rather thick, irregularly distanced and little engraved. Apertural end circular with a thin lip folded outwards. Septum well developed, oblique angled to the back of the shell. Maximum diameter, at aperture, is constant to middle of the teleoconch while the posterior diameter is always smaller. Operculum and soft parts not known.” di Geronimo et al. (1995: 114).

Description. Protoconch paucispiral (about 1.4 whorls), planispiral, smooth (Fig. 97A), transition to teleoconch I marked by very slight axial line (Fig. 97A). Teleoconchs I and II short, no sculpture observed, transition to teleoconch III marked by very slight axial line (Fig. 97A). Teleoconch III sculpture smooth with occasional axial growth striae (Fig. 97C), transition to teleoconch IV marked by incised axial line (Fig. 97B). Teleoconch IV with identical characters as previous stage, transition to teleoconch V marked by incised axial line (Fig. 97B). Teleoconch V (last growth stage) small for genus [Tol 1.26–1.45 mm], tubular, thin, very narrow, mildly to moderately and regularly arched [Larc: 0.45–0.55 mm; Arc: 0.13–0.14 mm], subcylindrical, semi-translucent to opaque white (Figs. 96 A–B). Surface smooth with numerous weak annuli (Fig. 97E). Posterior [Dpe: 0.20–0.22 mm] with squared shoulder (Fig. 97D). Septum mucronate (Fig. 97D). Mucro heel-like, positioned along dorsal margin, usually covered by septum in adult stage (Fig. 97D). Aperture [Da: 0.24–0.25 mm] thin (Fig. 97F). No varix. Lip smooth, with deflected peristome (Fig. 97F). Periostracum not observed. Operculum thin, light brown; exterior surface with 5–6 concentric rings (Fig. 97D). Conceptual reconstruction of growth stages shown in Fig. 98A.

Distribution and Habitat. From Westhoff Island, Chile to Stanley, East Falkland, Falkland Islands. Subtidal, 8– 245 m. Uncommon on organogenic sand substrates.

Remarks. The type material of Caecum magellanicum is comprised of three late subadult stages and nine adult specimens (holotype IPOP M1.8.8.95, Fig. 96E). Caecum magellanicum is the southern-most caecid known in the eastern Pacific and has also been found in the southern Atlantic.

Despite repeated attempts to obtain better images, the figures shown of the type material (Figs. 96E; 97A) are from di Geronimo et al. (1995). Hence it was not possible to observe the sculpture of teleoconchs I and II.