Caecum reversum Carpenter, 1857

Figures 42 A–F, 43A–H, 44A–B, 126K

Caecum (Fartulum) reversum Carpenter, 1857: 329, holotype NHMUK 1857.6.4.1549, Mazatlan.

Caecum (Fartulum) reversum — Carpenter, 1859: 434.

Caecum reversum — Carpenter, 1860: 33; Palmer, 1951: 44; Brann, 1966: pl. 37, fig. 377; Pizzini et al. 2007: 4, figs. 2a–d. Fartulum reversum — Keen, 1968: 420, fig. 94; Keen, 1971: 398; Kaiser, 1997: 29; Finet et al. 2016: 96.

Material examined. In addition to the type material the following lots were examined. NHMLAC: LACM 1977 - 144.14, Chame Point, Panama Bay, Panama Prov., Panama, 4 sh ; LACM 1984 - 45.11, between Seymour Island and Baltra Island, Galapagos Islands, 30–46 m, 4 sh ; LACM 1980 - 65.13, Salango Island, Manabi, Ecuador, 6–12 m, 5 sh ; LACM 1934 - 288.3, Tagus Cove, Albemarle Island, Galapagos Islands, 3 sh ; LACM 1933 - 163.4, Albemarle Point, Albemarle Island, Galapagos Islands, 26 m, 2 sh ; LACM 1984 - 46.10, Plazas Island, E of Santa Cruz Island, Galapagos Islands, 21–30 m, 1 sh ; LACM 1934 - 268.2, South Seymour Island, Galapagos Islands, 30 m, 1 sh ; LACM 1934 - 273.10, James Bay, James Island, Galapagos Islands, 44 m, 3 sh . SBMNH: SBMNH 620176, Barra de Navidad, Barra de Navidad, Jalisco, Mexico, 3–20 m, 2 sh ; SBMNH 619896, Playa Tamarindo, Guanacaste, Costa Rica, 6–15 m, 3 sh ; SBMNH 452448, Punta Chame, Panama Prov., Panama, 36–54 m, 6 sh ; SBMNH 452449, Balboa, Panama Prov., Panama, 20 sh ; SBMNH 452450, Isla Jaltemba, Nayarit, Mexico, 10–25 m, 40 sh ; SBMNH 452530, Punta Chame, Panama Prov., Panama, 40 sh ; SBMNH 452535, Balboa, Panama Prov., Panama, 15 sh ; SBMNH 452451, Bahia Audiencia, Manzanillo, Colima, Mexico, 10–25 m, 1 sh ; SBMNH 618585, Isla Seymour Norte, Isla Seymour Norte, Galapagos, Ecuador, 13–17 m, 5 sh ; SBMNH 620817, Punta Chame, Panama Prov., Panama, 12 sh .

Original diagnosis. “ C. ( Fartulum) t. juniore ‘ C. dextroverso ’ simili, sed septo subconvexo, mucrone reversa, semi-cylindracea, arcuata, aperturam versus convexa; dorsum versus convexa t. adulta?….; operculo?” Carpenter (1857: 329).

Description. Protoconch and teleoconch I unknown. Teleoconch II short, sculptured with weak longitudinal wrinkles (Fig. 43C), transition to teleoconch III marked by axial incised line (Fig. 43A). Teleoconch III about 2.5 times length of teleoconch II, with identical characters as previous stage (Fig. 43C), transition to teleoconch IV marked by axial constriction (Fig. 43D). Teleoconch IV sculptured with dense longitudinal striae (Fig. 43H), transition to teleoconch V marked by suture (Fig. 44A). Teleoconch V with identical characters as previous stage (Fig. 43H), transition to teleoconch VI marked by suture (Fig. 44A). Teleoconch VI (last growth stage) large for genus [Tol 3.14–3.16 mm], tubular, moderately and regularly arched [Larc: 1.31–1.39 mm; Arc: 0.27–0.28 mm], subcylindrical with slight increase in diameter from posterior to aperture, cream to opaque white (Figs. 42 A–B). Surface sculptured with dense longitudinal striae (Fig. 43H). Posterior [Dpe: 0.45 –0.48 mm] with rather thin edge, squared shoulder (Figs. 42 C–D). Septum mucronate, recessed (Figs. 42 C–D). Mucro irregular, blade shaped, position variable, curved, wavy, or folded toward margin (Figs. 42 C–D). Aperture [Da: 0.65–0.66 mm] thick (Fig. 42C). No varix. Lip smooth (Fig. 42C). Periostracum thin, smooth, yellowish (Fig. 42A). Operculum thin, light brown; exterior surface with 5–6 concentric rings (Fig. 42F). Conceptual reconstruction of growth stages shown in Fig. 44A.

Distribution and Habitat. From Navidad Bar, Jalisco, Mexico to Manabi, Ecuador, including the Galapagos Islands. Intertidal to 54 m. Common subtidally on sand and rubble substrates.

Remarks. Although Carpenter (1857) named Caecum reversum based on the mucro of the holotype curving backward, this is not always the case (NHMUK 1857.6.4.1549, Fig. 42E). The mucro of C. reversum may also be wavy or folded to one side (Figs. 42 C–D), and the position of the mucro can change with each subsequent growth stage (Figs. 43 A–B, D–G).