Caecum diminutum C.B. Adams, 1852

Figures 3 A–I, 4A–M, 5A–B, 6N–O, 127P

Caecum diminutum C.B. Adams, 1852: 161, holotype MCZ 186574, Panama.

Caecum (Fartulum) farcimen Carpenter, 1857 (in part): 326, 4 syntypes NHMUK 1857.6.4.1544, 1 syntype NHMUK 1857.6.4.1545, Mazatlan. (syn. nov.).

Caecum impartitum var. bicolor de Folin, 1867a: 41, 3 syntypes MNHN-IM-2000-4600, Panama. (syn. nov.).

Caecum taeniatum de Folin, 1867b: 43, 3 syntypes MNHN-IM-2000-3115, Panama. (syn. nov.).

Caecum diminutum — Turner, 1956: 45, pl. 17, fig. 5; Keen, 1971: 397; Lightfoot, 1993a: 18, fig. 7.

Caecum farcimen — Carpenter, 1860: 32; Tryon, 1886: 215, pl. 66, fig. 53; Palmer, 1951: 44; Keen, 1968: 418, figs. 78a–b; Keen, 1971: 397; Shasky, 1984: 27; Skoglund, 1992: 28.

non Brochina farcimen — Lightfoot, 1993b: 80, fig. 27.

Caecum sp. “ a ”— Cruz, 2007: 135, fig. 7.

Material examined. In addition to the type material the following lots were examined. NHMLAC: LACM 1986 - 195.6, La Gringa Point, Los Angeles Bay, Baja California, Mexico, 5 m, 6 sh ; LACM 1946 - 10.12, Creston Island, Mazatlan, Sinaloa, Mexico, intertidal, 1 sh ; LACM 1972 - 42.64, Ballena Point, Ballena Bay, Puntarenas Prov., Costa Rica, 3–17 m, 30 sh ; LACM 1975 - 54.26, Venado Island, Panama Prov., Panama, intertidal, 3 sh ; LACM 1933 - 10.13, Santa Elena Beach, Guayas Prov., Ecuador, intertidal, 10 sh; LACM 1970 - 11.52, Ancon Point, Guayas Prov., Santa Elena, Ecuador, intertidal, 17 sh; LACM 1970 - 12.41, Ancon Point, Guayas Prov., Santa Elena, Ecuador, intertidal, 87 sh; LACM 1935 - 106.39, Blanca Beach, Guanacaste Prov., Costa Rica, intertidal, 1 sh; LACM 1967 - 12.14, Isabel Island, Nayarit, Mexico, 24 m, 3 sh ; LACM 1965 - 65.2, Caraya Island, Panama Bay, Panama Prov., Panama, intertidal, 1 sh ; LACM 182097, Taboga Island, Panama Bay, Panama Prov., Panama, intertidal, 2 sh ; LACM 1965 - 25.56, Taboga Island, Panama Bay, Panama Prov., Panama, 9–27 m, 11 sh ; LACM 1938 - 131.20, Honda Bay, Veraguas Prov., Panama, intertidal to 17 m, 2 sh ; LACM 1972 - 22.14, San Pedrito Island, Murcielagos Islands, Guanacaste Prov., Costa Rica, 3–4 m, 1 sh ; LACM 1972 - 46.61, Alcatraz Island, Tortugas Islands, Puntarenas Prov., Costa Rica, 2–8 m, 120 sh ; LACM 1972 - 52.66, Herradura Bay, Puntarenas Prov., Costa Rica, 3–16 m, 50 sh ; LACM 1935 - 122.18, Salinas Bay, Costa Rica, 10 m, 4 sh ; LACM 1972 - 17.44, Jobo Bay, Costa Rica, 2–8 m, 100 sh ; LACM 1972 - 58.61, Culebra Point, Costa Rica, 10–25 m, 27 sh ; LACM 1970 - 9.30, Salinas, Ecuador, intertidal, 95 sh; LACM 1972 - 85.33, Mero Pt., Tumbes Dept., Peru, intertidal, 7 sh; LACM 1973 - 57.6, Zacatillo Island, El Salvador, intertidal, 30 sh ; LACM 1973 - 95.2, Los Cobanos, El Salvador, 2 m, 65 sh ; LACM 1972 - 81.19, S of Bocapan, Tumbes Dept., Peru, intertidal, 1 sh . SBMNH: SBMNH 620188, La Puntilla, Salinas, Guayas, Ecuador, intertidal, 20 sh; SBMNH 619893, La Puntilla, Salinas, Guayas, Ecuador, intertidal, 1 sh; SBMNH 620178, Sayulita, Nayarit, Mexico, intertidal, 1 sh; SBMNH 620187, Bahia Herradura, Puntarenas, Costa Rica, intertidal, 1 sh ; SBMNH 621307, La Cruz de Huanacaxtle, Bahia de Banderas, Nayarit, Mexico, intertidal, 3 sh ; SBMNH 621313, Punta Rosales, Jalisco, Mexico, intertidal, 5 sh; SBMNH 128133, Isla Lobos, Mazatlan, Sinaloa, Mexico, intertidal, 24 sh; SBMNH 126597, Playa Las Gaviotas, Mazatlan, Sinaloa, Mexico, intertidal, 24 sh; SBMNH 621401, Punta Jacinto, Guayas, Ecuador, intertidal, 3 sh; SBMNH 620756, La Cruz de Huanacaxtle, Nayarit, Mexico, intertidal, 40 sh ; SBMNH 620692, Lo de Marcos, Nayarit, Mexico, intertidal, 30 sh ; SBMNH 620849, Punta Tiburon, Mazatlan, Sinaloa, Mexico, intertidal, 2 sh; SBMNH 620863, Punta Ursula, Veraguas, Panama, 5–10 m, 1 sh ; SBMNH 620867, La Cruz de Huanacaxtle, Nayarit, Mexico, 9–14 m, 6 sh ; SBMNH 621362, Santa Cruz, Nayarit, Mexico, intertidal, 10 sh; SBMNH 621645, Sayulita, Nayarit, Mexico, intertidal, 1 sh; SBMNH 622338, El Rubio, Tumbes, Peru, intertidal, 1 sh .

Original diagnosis. “Shell well arcuated, slender, with the diameter uniform, except near the summit: white: with about 17 stout rings, which are rather distant and are but slightly flattened at the summits, followed by about 10 rings which are small and less prominent: apex lateral, not prominent.” Adams (1852: 161).

Description. Protoconch and teleoconch I unknown. Teleoconch II short, sculptured with axial growth lines increasing in strength, transition to teleoconch III marked by axial incised line followed by increase in diameter (Fig. 4A). Teleoconch III about 2.5 times length of teleoconch II, smooth or sculptured with narrow rudimentary ribs and short longitudinal striae (Figs. 4C, E, F), transition to teleoconch IV marked by a slight axial constriction (Figs. 4A, D, G). Teleoconch IV with identical characters as previous stage, transition to teleoconch V marked by a rapid increase in diameter (Figs. 4A, D, G). Teleoconch V (last growth stage) small for genus [Tol 1.40–1.68 mm], tubular, robust, strongly and regularly arched [Larc: 0.40–0.44 mm; Arc: 0.14–0.18 mm], nearly uniform in diameter from posterior to aperture, cream to semi-translucent brown, or mottled (Figs. 3 A–C). Surface smooth or sculptured with numerous, wavy longitudinal striae in the interspaces (Figs. 4K, M). Axial sculpture from nearly absent to consisting of 20–30 ribs, which vary greatly in strength and spacing (Figs. 4H, K, M). Axial interspaces vary greatly in width, rather shallow (Fig. 4L). Posterior [Dpe: 0.34–0.39 mm] with moderately thick edge (Fig. 3F). Septum mucronate (Fig. 3F). Mucro with low rounded point, positioned along dorsal margin (Fig. 3F). Aperture [Da: 0.42–0.45 mm] simple, slightly constricted (Figs. 4H, K, M). No varix. Lip smooth, weakly developed, with slightly deflected peristome (Fig. 3F). Periostracum not observed. Operculum thin, light brown; exterior surface with 8–10 concentric rings (Figs. 3 G–H). Conceptual reconstruction of growth stages shown in Fig. 5A.

Distribution and Habitat. From Sinaloa, Mexico to Tumbes, Peru. Intertidal to 27 m. Common intertidally on rocky substrates.

Remarks. The holotype of Caecum diminutum (MCZ 186574) is an adult specimen (Fig. 3F, I). Width of axial interspaces varies greatly. At some points they are so narrow they appear almost absent even within a single population. Neither locality nor depth appears to influence the surface sculpture. Carpenter (1857) was unaware of this variability when describing C. farcimen . Although the four specimens on Carpenter’s tablet 1544 are subadult (NHMUK 1857.6.4.1544, Fig. 3J), they clearly represent the smoother form of C. diminutum . The remaining C. farcimen type specimen (NHMUK 1857.6.4.1545) found on tablet 1545 is actually a junior synonym of another species, which is discussed later. Ribbed forms of C. diminutum can appear somewhat similar to C. firmatum Adams, 1852 . In fact, Carpenter (1863) considered it to be a synonym. However, differences in growth morphology and microsculpture do not support this position. Lightfoot (1993a: 18) correctly identified and illustrated C. diminutum . However, in the second part of her work, Lightfoot (1993b: 80) considered C. farcimen to be a separate unrelated species. De Folin (1867a,b) overlooked C. diminutum when describing C. impartitum var. bicolor (Fig. 3K) and C. taeniatum (Fig. 3L).