Figures 6 A–G, 7A–I, 8A–B, 127J
Caecum firmatum C.B. Adams, 1852: 161, lectotype (Turner, 1956) MCZ 186578, Panama.
Caecum eburneum C.B. Adams, 1852: 161, lectotype (Turner, 1956) MCZ 186575, paralectotype MCZ 186576, Panama, [non de Folin, 1886].
Caecum monstrosum C.B. Adams, 1852: 162, lectotype (Turner, 1956) MCZ 186584, Panama.
Caecum pygmaeum C.B. Adams, 1852: 163, holotype MCZ 186587, Panama.
Caecum agoniatum de Folin, 1867a: 42, 6 syntypes MNHN-IM-2000-4579, Panama. (syn. nov.).
Caecum elegans var. rubella de Folin, 1867a: 45, 8 syntypes MNHN-IM-2000-4580, Panama. (syn. nov.).
Caecum impartitum de Folin, 1867a: 41, 57 syntypes MNHN-IM-2000-4599, Panama. (syn. nov.).
Caecum suave de Folin, 1869: 132, pl. XX, figs. 1–2, holotype MNHN-IM-2000-4585, Panama. (syn. nov.).
Caecum firmatum — Carpenter, 1860: 32; Carpenter, 1863: 350; Tryon, 1886: 217, pl. 67, fig. 66; Palmer, 1951: 43; Hertlein & Strong, 1955: 136; Turner, 1956: 49, pl. 17, fig. 6; Keen, 1971: 397; Shasky, 1984: 27; Skoglund, 1992: 28; Lightfoot, 1993a: 17, fig. 5; Kaiser, 1997: 29; Finet et al. 2016: 28.
Caecum eburneum — Carpenter, 1863: 350; Turner, 1956: 47, pl. 17, fig. 3; Keen, 1971: 397; Skoglund, 1992: 28; Lightfoot, 1993a: 19, fig. 6; Kaiser, 1997: 29; Finet et al. 2016: 28.
Caecum monstrosum — Carpenter, 1863: 350; Turner, 1956: 65, pl. 17, fig. 2; Keen, 1971: 397.
Caecum pygmaeum — Carpenter, 1863: 350; Turner, 1956: 79, pl. 17, fig. 1; Keen, 1971: 397; Lightfoot, 1993a: 20, fig. 9.
Material examined. In addition to the type material the following lots were examined. NHMLAC: LACM 1976 - 7.13, La Gringa Point, Los Angeles Bay, Baja California, Mexico, 34 m, 60 sh; LACM 1973 - 3.28, Coyote Bay, Baja California Sur, Mexico, 9–22 m, 100+ sh; LACM 1936 - 187.2, Escondido Port, Baja California Sur, Mexico, 9 m, 75 sh; LACM 1972 - 42.65, Ballena Point, Ballena Bay, Costa Rica, 3–17 m, 24 sh; LACM 1972 - 17.45, Jobo Bay, Salinas, Costa Rica, 1–7 m, 65 sh; LACM 1972 - 46.62, Alcatraz Island, Tortugas Islands, Costa Rica, 2–7 m, 100+ sh; LACM 1972 - 63.84, Cano Island, Costa Rica, 8–13 m, 4 sh; LACM 1946 - 10.13, Creston Island, Mazatlan, Sinaloa, Mexico, intertidal, 2 sh; LACM 1965 - 14.33, Maria Cleofas Island, Tres Marietas Islands, Mexico, 5–10 m, 4 sh; LACM 1967 - 9.10, Isabel Island, Nayarit, Mexico, 10 m, 2 sh; LACM 1967 - 15.14, Banderas Bay, Jalisco, Mexico, 10–20 m, 22 sh; LACM 1973 - 6.47, Venado Island, Guaymas, Sonora, Mexico, intertidal, 12 sh; LACM 1975 - 54.27, Venado Island, Panama, intertidal, 40 sh; LACM 1975 - 55.31, San Carlos, Panama, intertidal, 24 sh; LACM 1964 - 36.48, San Carlos Bay, Guaymas, Sonora, Mexico, 20–40 m, 12 sh; LACM 1967 - 17.44, Libertad Port, Sonora, Mexico, intertidal, 22 sh; LACM 1966 - 28.29, Partida Bay, Baja California, Mexico, intertidal to 11 m, 6 sh; LACM 1974 - 86.13, El Velero, Nicaragua, 2 m, 10 sh; LACM 1966 - 12.50, Cape San Lucas, Baja California, Mexico, 8–33 m, 1 sh; LACM 1965 - 21.63, Otoque Island, Panama Bay, Panama, 9–27 m, 4 sh; LACM 1934 - 287.11, Academy Bay, Indefatigable Island, Galapagos Islands, 27 m, 8 sh; LACM 1965 - 12.42, Maria Cleofas Island, Tres Marias Islands, Mexico, 27–55 m, 28 sh; LACM 1966 - 19.61, Pulmo Bay, Baja California Sur, Mexico, 2–7 m, 6 sh; LACM 1962 - 22.30, Cholla Bay, Port Penasco, Sonora, Mexico, intertidal, 40 sh; LACM 1965 - 34.49, Puertecitos, Baja California, Mexico, intertidal to 10 m, 200 sh; LACM 1961 - 2.7, San Carlos Bay, Guaymas, Sonora, Mexico, intertidal, 60 sh; LACM 1966 - 20.22, S end of Pulmo Bay, Baja California Sur, Mexico, 7 m, 100+ sh; LACM 1968 - 14.24, Estero Soldado, NW of Guaymas, Sonora, Mexico, intertidal to 3 m, 58 sh; LACM 1968 - 21.4, Barra de Navidad, Jalisco, Mexico, intertidal, 8 sh; LACM 1966 - 16.33, El Tule, Canelo Bay, Baja California Sur, Mexico, 5–10 m, 21 sh. SBMNH: SBMNH 619600, Isla Maria Madre, SE Cabo Alto, Nayarit, Mexico, 15–20 m, 4 sh; SBMNH 621639, Majahuitas, Jalisco, Mexico, 3–15 m, 2 sh; SBMNH 452406, Roca Acero, Sonora, Mexico, 28 m, 8 sh; SBMNH 452412, Roca Acero, Sonora, Mexico, 28 m, 1 sh; SBMNH 619586, Playa Tamarindo, Guanacaste, Costa Rica, 6–15 m, 1 sh; SBMNH 620629, Caleta Lalo, Sonora, Mexico, 15 m, 12 sh; SBMNH 620672, Bahia Bocochibampo, Sonora, Mexico, intertidal, 2 sh; SBMNH 620759, Bahia Bocochibampo, Sonora, Mexico, intertidal, 20 sh; SBMNH 620847, Bahia Bocochibampo, Sonora, Mexico, intertidal, 2 sh; SBMNH 620447, Playa Kobbe, Panama Prov., Panama, intertidal, 20 sh; SBMNH 452399, Punta Coyote, Baja California Sur, Mexico, 20–57 m, 2 sh; SBMNH 620805, Bahia Bocochibampo, Sonora, Mexico, 0–2 m, 15 sh; SBMNH 620872, Bahia Bocochibampo, Sonora, Mexico, 0–2 m, 7 sh; SBMNH 621399, Punta Jacinto, Guayas, Ecuador, intertidal, 30 sh; SBMNH 620241, Santa Cruz, Nayarit, Mexico, intertidal, 12 sh; SBMNH 620240, Bahia Cuastecomate, Jalisco, Mexico, 12–30 m, 12 sh; SBMNH 622320, Punta Mala, Manabi, Ecuador, intertidal, 1 sh; SBMNH 621403, Estero Morua, Sonora, Mexico, intertidal, 12 sh; SBMNH 351449, Lions Rock, San Luis Obispo County, California, USA, 28 m, 5 sh; SBMNH 620774, Isla Venado, Panama Prov., Panama, intertidal, 10 sh; SBMNH 19961, Puerto Vallarta, Jalisco, Mexico, 9 sh; SBMNH 620586, Bahia Cuastecomate, Jalisco, Mexico, 10–15 m, 12 sh; SBMNH 620673, Bahia Cuastecomate, Jalisco, Mexico, 10–25 m, 1 sh; SBMNH 126318, Puertecitos, 1.5 mi N [Playa Curbina], Baja California [Norte], Mexico, 40 sh; SBMNH 126315, Puertecitos, 1.5 mi N [Playa Curbina], Baja California [Norte], Mexico, 10 sh; SBMNH 215012, Puertecitos, 1 mi N, Baja California [Norte], Mexico, 3 sh; SBMNH 126316, Puertecitos, 1 mi N, Baja California [Norte], Mexico, 25 sh; SBMNH 214936, Puertecitos, 1 mi N, Baja California [Norte], Mexico, 1 sh; SBMNH 215003, Puerto Penasco, west bight, Sonora, Mexico, 50 sh; SBMNH 127634, Bahia San Luis Gonzaga, N entrance, Baja California [Norte], Mexico, 4–7 m, 1 sh; SBMNH 620596, Bahia San Luis Gonzaga, N entrance, Baja California [Norte], Mexico, 4–6 m, 1 sh; SBMNH 620610, Los Islotes, Baja California Sur, Mexico, 12 m, 1 sh; SBMNH 110189, Acapulco, Guerrero, Mexico, 1–2 m, 25 sh; SBMNH 110190, Acapulco, Guerrero, Mexico, 1–2 m, 1 sh; SBMNH 215001, Marua, Sonora, Mexico, intertidal, 1 sh; SBMNH 214990, Bahia El Dorado, Manzanillo, Colima, Mexico, 10–25 m, 4 sh; SBMNH 215019, Bahia San Francisco, San Carlos Nuevo Guaymas, Sonora, Mexico, 1 sh; SBMNH 452393, Isla San Jorge [Bird Island], 5–7 mi SW of, Sonora, Mexico, 0–5 m, 1 sh; SBMNH 452395, Isla Danzante, W of, Baja California Sur, Mexico, 50–57 m, 3 sh; SBMNH 452396, Punta Mareno, Sonora, Mexico, 10–25 m, 4 sh; SBMNH 452397, Punta Doble, San Carlos Nuevo Guaymas, Sonora, Mexico, 10–25 m, 12 sh; SBMNH 452398, Isla Danzante, Baja California Sur, Mexico, 60–90 m, 10 sh; SBMNH 452401, Ensenada San Francisco, Sonora, Mexico, 23–40 m, 8 sh; SBMNH 452402, Punta San Pedro, Sonora, Mexico, 10–25 m, 200 sh; SBMNH 452403, La Puntilla, Salinas, Guayas, Ecuador, intertidal, 18 sh; SBMNH 452404, Las Cocinas, Sonora, Mexico, 14 m, 300 sh; SBMNH 452405, Bahia Audiencia, Manzanillo, Colima, Mexico, 10–25 m, 2 sh; SBMNH 452407, Las Hadas, Colima, Mexico, 1–3 m, 20 sh; SBMNH 452408, Bahia Santiago, Colima, Mexico, 27–40 m, 20 sh; SBMNH 452409, Bahia Carrizal, Colima, Mexico, 0–3 m, 50 sh; SBMNH 452410, Punta Mareno, Sonora, Mexico, 10–25 m, 2 sh; SBMNH 452411, Isla San Jorge [Bird Island], 5–7 mi SW of, Sonora, Mexico, 20–30 m, 10 sh; SBMNH 452413, Roca Acero, Sonora, Mexico, 30–45 m, 6 sh; SBMNH 452442, Isla Danzante, NE end, Baja California Sur, Mexico, 67–100 m, 25 sh; SBMNH 452506, Bahia Santiago, Colima, Mexico, 27–40 m, 12 sh; SBMNH 452532, Las Hadas, Colima, Mexico, 1–3 m, 5 sh; SBMNH 15739, Bahia Cuastecomate, Jalisco, Mexico, 9 m, 1 sh; SBMNH 619532, Bahia Ballena, Puntarenas, Costa Rica, 13–15 m, 10 sh; SBMNH 619580, Bahia Santiago, Colima, Mexico, 30–60 m, 3 sh; SBMNH 619581, Estero Morua, Sonora, Mexico, 7–10 m, 50 sh; SBMNH 619582, Punta San Pedro, Sonora, Mexico, 10–25 m, 11 sh; SBMNH 132634, Tetas de Cabra, Sonora, Mexico, intertidal, 30 sh; SBMNH 126602, Playa Las Gaviotas, Mazatlan, Sinaloa, Mexico, intertidal, 1 sh; SBMNH 619585, Bahia Herradura, Puntarenas, Costa Rica, intertidal, 2 sh; SBMNH 619589, Estero Morua, Sonora, Mexico, intertidal, 1 sh; SBMNH 619594, Las Cocinas, Sonora, Mexico, 13 m, 20 sh; SBMNH 619792, Isla Jaltemba, Nayarit, Mexico, 10–25 m, 10 sh; SBMNH 619886, Isla Lobos, Mazatlan, Sinaloa, Mexico, intertidal, 2 sh; SBMNH 620167, Puerto Penasco, west bight, Sonora, Mexico, 50 sh; SBMNH 620170, Tisate, Bahia Banderas, Jalisco, Mexico, intertidal, 15 sh; SBMNH 15729, Isla Carmen, Baja California Sur, Mexico, 2 sh; SBMNH 620229, Punta Mita, Nayarit, Mexico, intertidal, 40 sh; SBMNH 620230, Isla San Juanito, near Roca Blanca, Nayarit, Mexico, 8–11 m, 1 sh; SBMNH 620231, Majahuitas, Jalisco, Mexico, 10–14 m, 1 sh; SBMNH 123192, Bahia Sebastian Vizcaino, Baja California [Norte], Mexico, 34 m, 12 sh; SBMNH 116693, Punta Colorada, Baja California Sur, Mexico, intertidal, 15 sh; SBMNH 119738, Punta Colorada, Baja California Sur, Mexico, intertidal, 50 sh; SBMNH 127138, Chacala, Nayarit, Mexico, 3 m, 1 sh; SBMNH 114035, Isla Natividad, opposite Flat Rock, Baja California Sur, Mexico, intertidal, 1 sh; SBMNH 116248, Bahia Tortugas, Baja California Sur, Mexico, 10–25 m, 1 sh; SBMNH 137376, Guaymas, Sonora, Mexico, 99–108 m, 4 sh; SBMNH 620235, Manzanillo, Colima, Mexico, 13–22 m, 2 sh; SBMNH 620236, Los Arcos, Jalisco, Mexico, 10–20 m, 2 sh; SBMNH 620237, Manzanillo, Colima, Mexico, 3 sh; SBMNH 620255, Punta Colorada, Baja California Sur, Mexico, intertidal, 100 sh; SBMNH 620449, Bahia San Francisco, San Carlos Nuevo Guaymas, Sonora, Mexico, 3–6 m, 20 sh; SBMNH 620452, Puertecitos, Baja California [Norte], Mexico, 6 sh; SBMNH 133397, Santa Barbara Island, Santa Barbara County, California, USA, 10 sh; SBMNH 133398, Santa Barbara Island, Santa Barbara County, California, USA, 6 sh; SBMNH 127144, Chacala, Nayarit, Mexico, 3–5 m, 6 sh; SBMNH 129731, Agua de Chale, Baja California [Norte], Mexico, intertidal, 6 sh; SBMNH 620559, San Blas, El Rey [light house island], Nayarit, Mexico, 3–4 m, 1 sh; SBMNH 620615, Isla Perico, Panama Prov., Panama, intertidal, 4 sh; SBMNH 620617, Veracruz, Panama Prov., Panama, intertidal, 3 sh; SBMNH 620622, Bahia San Carlos, Sonora, Mexico, 18–36 m, 30 sh; SBMNH 620624, Los Islotes, Baja California Sur, Mexico, 12 m, 2 sh; SBMNH 620627, Bahia Santa Cruz, Oaxaca, Mexico, 2–4 m, 1 sh; SBMNH 620628, Bahia Bocochibampo, Sonora, Mexico, 15–17 m, 5 sh; SBMNH 620637, Bahia Saladita, Sonora, Mexico, 2–4 m, 12 sh; SBMNH 620638, Isla Pajeros, Jalisco, Mexico, intertidal, 15 sh; SBMNH 620647, Bahia San Carlos, Sonora, Mexico, 15–17 m, 20 sh; SBMNH 620655, Isla Venado, Panama Prov., Panama, intertidal, 15 sh; SBMNH 620656, Bahia Saladita, Sonora, Mexico, 12 m, 2 sh; SBMNH 620657, Islas Ladrones, Chiriquí, Panama, 10–15 m, 1 sh; SBMNH 620658, Bahia Bocochibampo, Sonora, Mexico, intertidal, 4 sh; SBMNH 620659, Lo de Marcos, Nayarit, Mexico, intertidal, 10 sh; SBMNH 620660, Bahia San Carlos, San Carlos Nuevo Guaymas, Sonora, Mexico, 10–25 m, 1 sh; SBMNH 620666, Bahia San Carlos, San Carlos Nuevo Guaymas, Sonora, Mexico, 10–25 m, 30 sh; SBMNH 620683, Isla Pedro Gonzalez, Balboa District, Panama Prov., Panama, intertidal, 7 sh; SBMNH 620685, Sayulita, Nayarit, Mexico, 3–6 m, 1 sh; SBMNH 620693, Cabo Pulmo, Baja California Sur, Mexico, 8 m, 20 sh; SBMNH 620757, Punta Colorado, Sonora, Mexico, 18 m, 1 sh; SBMNH 620761, Bahia Tenacatita, Cabo Tamarino, Jalisco, Mexico, 8–10 m, 6 sh; SBMNH 620780, Isla Pajeros, Jalisco, Mexico, intertidal, 25 sh; SBMNH 620783, Bahia Saladita, Sonora, Mexico, 12 m, 20 sh; SBMNH 620785, Bahia San Carlos, San Carlos Nuevo Guaymas, Sonora, Mexico, 60–90 m, 1 sh; SBMNH 620848, Isla Venado, Panama Prov., Panama, intertidal, 8 sh; SBMNH 115318, Ocean Beach, San Diego County, California, USA, 10–25 m, 1 sh; SBMNH 128456, Bahia Salinas, Baja California Sur, Mexico, 5–18 m, 40 sh; SBMNH 116241, Bahia Tortugas, Baja California Sur, Mexico, 5 m, 5 sh; SBMNH 621297, Isla El Requeson, Bahia Concepcion, Baja California Sur, Mexico, intertidal, 3 sh; SBMNH 621319, Playa Cochorit, Sonora, Mexico, 30 sh; SBMNH 621328, Bahia San Carlos, San Carlos Nuevo Guaymas, Sonora, Mexico, 40 sh; SBMNH 621329, Playa Venado, Panama Prov., Panama, 15 sh; SBMNH 621331, Bahia Algodones, San Carlos Nuevo Guaymas, Sonora, Mexico, intertidal, 25 sh; SBMNH 621347, Bahia San Carlos, San Carlos Nuevo Guaymas, Sonora, Mexico, 15–30 m, 4 sh; SBMNH 621348, Puerto Lobos, Cabo Tepoca, Sonora, Mexico, intertidal, 30 sh; SBMNH 621363, Bahia Herradura, Puntarenas, Costa Rica, intertidal, 4 sh; SBMNH 621426, Punta Mita, Nayarit, Mexico, intertidal, 3 sh; SBMNH 131983, Point Loma, San Diego County, California, USA, 33 m, 2 sh; SBMNH 621638, Islas Tres Marias, Nayarit, Mexico, 3–7 m, 1 sh; SBMNH 621643, Bahia Santiago, Colima, Mexico, 15–30 m, 1 sh; SBMNH 621668, Bahia San Francisco, San Carlos Nuevo Guaymas, Sonora, Mexico, 3 sh; SBMNH 621671, Isla San Juanito, near Roca Blanca, Nayarit, Mexico, 8–11 m, 1 sh; SBMNH 622326, Playas de Villamil, Guayas, Ecuador, intertidal, 1 sh; SBMNH 622330, Isla Pedro Gonzalez, Balboa District, Panama Prov., Panama, intertidal, 12 sh; SBMNH 622339, Isla Taboga, Panama Prov., Panama, 10 sh; SBMNH 622359, Punta Ancon, Guayas, Ecuador, intertidal, 10 sh; SBMNH 622384, Isla Venado, Panama Prov., Panama, intertidal, 10 sh; SBMNH 622487, El Rubio, Tumbes, Peru, intertidal, 1 sh; SBMNH 622506, Bahia Santiago, Colima, Mexico, 13 m, 10 sh; SBMNH 622507, Bahia Carrizal, Colima, Mexico, 0–3 m, 3 sh; SBMNH 622508, Isla Chapetona, Sonora, Mexico, 0–2 m, 50 sh; SBMNH 622509, Punta San Antonio, Sonora, Mexico, 90 m, 12 sh. CAS: CASIZ 228930, Albemarle Island, Galapagos Islands, 11 sh; CASIZ 228931, Indefatigable Island, Galapagos Islands, 12 sh. BCD: Navidad Bar, Colima, Mexico, 16–30 m, 14 sh; Piedras Blancas, Nayarit, Mexico, 18 m, 13 sh; Cholla Bay, Penasco Point, Sonora, Mexico, intertidal, 9 sh.
Original diagnosis. “Shell rather stout, well arcuated, widening towards the aperture, moderately contracted at the summit: ivory white: with about 25 very stout broad prominent not closely approximate rings, with flat summits: apex not very prominent. Length.09 inch; breadth.03 inch. A variety (?) is more slender, and has the ribs less prominent.” Adams (1852: 161).
Description. Protoconch multispiral (about 1.75 whorls), planispiral, smooth (Fig. 7A), transition to teleoconch I marked by apertural varix (Fig. 7A). Teleoconch I short, sculptured with weak axial threads, crossed by irregular longitudinal sculpture, creating a reticular pattern (Fig. 7B), transition to teleoconch II marked by slight axial constriction followed by change in sculpture (Figs. 7A, D, G). Teleoconch II about the same length of teleoconch I, sculptured with irregular longitudinal sculpture, crossed by low axial annuli (Fig. 7C), transition to teleoconch III marked by change in sculpture (Fig. 7D). Teleoconch III sculptured with axial annuli, crossed by short longitudinal striae (Fig. 7E), transition to teleoconch IV marked by change in sculpture (Figs. 7D, G). Teleoconch IV sculptured with axial ribs, longitudinal striae within interspaces only (Fig. 7F), transition to teleoconch V marked by suture, immediately followed by pronounced axial ribs (Figs. 7D, G). Teleoconch V (last growth stage) average size for genus [Tol 2.10–2.56 mm], tubular, thick, robust, moderately and regularly arched [Larc: 0.62–0.77 mm; Arc: 0.20–0.28 mm], subcylindrical with increase in diameter from posterior to aperture, cream with brownish mottling (Figs. 6 A–B). Surface sculptured with axial ribs and longitudinal striae within the interspaces (Figs. 7 H–I). Striae thin, but dense (Fig. 7H). Axial sculpture comprising of 24–34 raised, strong, narrow to wide, quadrangular ribs, rather closely arranged and regularly spaced (Fig. 7I), except last 3–4 preceding aperture, which are slightly larger (Figs. 6 A–B, F; 7I). Axial interspaces moderately deep, narrower than ribs (Fig. 7I), except last rib preceding apertural ribs, which is somewhat wider (Fig. 7I). Posterior [Dpe: 0.45–0.53 mm] with thick edge, squared shoulder (Fig. 7I). Septum mucronate (Fig. 7I). Mucro with low rounded point, positioned along dorsal margin (Fig. 7I). Aperture [Da: 0.59–0.70 mm] with 3–4 large axial ribs, last one to two sharply reduced in width on ventral side (Fig. 7I). No varix. Lip smooth, with deflected peristome (Fig. 6C). Periostracum not observed. Operculum thin, light brown; exterior surface with 6–8 concentric rings (Figs. 6 D–E). Conceptual reconstruction of growth stages shown in Fig. 8A.
Distribution and Habitat. From Cholla Bay, Sonora, Mexico to Salinas, Ecuador including the Galapagos Islands. Intertidal to 100 m. Abundant subtidally on coarse sand and rubble substrates.
Remarks. The type material of Caecum firmatum is comprised of three adults and one subadult (lectotype MCZ 186578, Figs. 6 F–G). Carpenter (1863) published a review of Adam’s type specimens from Panama, where he considered C. diminutum, C. eburneum, C. monstrosum and C. pygmaeum to be conspecific with C. firmatum. However, from the citations listed above it would appear Carpenter’s opinion has either been overlooked or ignored for the most part. Yet a comparison of the type specimens via SEM imaging confirms Carpenter’s assessment to be correct for all but one species. As previously stated, ribbed forms of C. diminutum (Figs. 6 N–O) may appear similar to C. firmatum, but the differences in microsculpture and growth morphology do not support Carpenter’s position.
Caecum firmatum develops several larger apertural axial ribs that are separated from the rest by a somewhat wider interspace (Fig. 7I), but it lacks a true sulcus or varix. The most similar congener is C. quadratum Carpenter, 1857, which can be confused when specimens are worn. Both species are quite variable, which has resulted in many synonyms. Axial rib thickness of C. firmatum is quite variable. From 1867 to 1869, de Folin described several species: C. agoniatum, C. elegans var. rubella, C. impartitum, and C. suave (Figs. 6 P–S), which fall within the species variability of C. firmatum.
The holotype of Caecum pygmaeum (MCZ 186587, Figs. 6 J–K) may at first appear similar to C. clathratum Carpenter, 1857. However, C. pygmaeum displays more numerous flattened ribs and lacks the concave interspaces present in C. clathratum. The type specimen of C. pygmaeum is most likely a heavily eroded C. firmatum as synonymized by Carpenter.
There seems to be some confusion regarding the identification of certain caecids found around the Galapagos Islands (Finet et al. 2016: 28). To resolve this issue, examinations of the specific lots in question were completed. Caecum firmatum is now confirmed to be among those species from the Galapagos Islands as identified by Strong, and recorded by Hertlein & Strong (1955: 136).
Lastly, Caecum eburneum de Folin, 1886, is an unrelated species from Australia, which Kisch (1959b) renamed C. folini, because de Folin’s name was preoccupied.