Meioceras nitidum, (STIMPSON, 1851)

Raines, Bret, 2023, Caecidae (Mollusca: Gastropoda) from late Miocene exposures of the “ Imperial ” Formation in Riverside County, California, PaleoBios 40 (1928), pp. 1-11 : 8-9

MEIOCERAS NITIDUM ( STIMPSON, 1851)

FIG. 7A–C

Caecum nitidum Stimpson, 1851: 112, (original type material destroyed), Florida .

Caecum (Meioceras) nitidum — Dall, 1892: 302; Morse, 1919: 76, pl. V, fig. 6; Moore, 1972: 892, fig. 11; Abbott, 1974: 94, fig. 895; Keller, 1981: 71, fig. 22; Vokes & Vokes, 1983: 16; Mello & Maestrati, 1986: 161, fig. 16; Lightfoot, 1992: 29, fig. 33; Rios, 1994: 58, pl. 19, fig. 217; Bandel, 1996: 63, pl. 5, fig. 1–6; Gomes & Absalão, 1996: 524, fig. 14; Oliveira & Almeida, 1999: 3, fig. 13; Daccarett and Bossio 2011: 73, fig. 244.

Meioceras nitidum ( Stimpson, 1851) — Gomes, 1999: 27; Redfern, 2013: 71, fig. 211A–E; Lester, 2017: 27, fig 9; Lamy & Pointier, 2017: 203, fig. 12, Egger et al. 2020: 14 , fig. 5A–D; Costa et al. 2021: 84, fig. 2I.

Caecum rotundum de Folin, 1868: 49 , pl. 5, fig. 2 — Lester, 2017: 27.

Meioceras bitumidum de Folin, 1869a: 25 , fig. 4 — Kisch, 1959: 39; Moore, 1972: 892; Abbott, 1974: 94; Gomes, 1999: 19, fig. 14; Lester, 2017: 27.

Meioceras carpenteri de Folin, 1869a: 24 , fig. 3 — Kisch, 1959: 39; Moore, 1972: 892; Abbott, 1974: 94; Gomes, 1999: 20, fig. 15; Lester, 2017: 27.

Meioceras coxi de Folin, 1869a: 29 , fig.9 — Kisch, 1959: 39; Moore, 1972: 892; Abbott, 1974: 94; Gomes, 1999: 20, fig. 16; Lester, 2017: 28.

Meioceras crossei de Folin, 1869a: 27 , fig. 7 — Kisch, 1959: 39; Moore, 1972: 892; Abbott, 1974: 94; Gomes, 1999: 21, fig. 17.

Meioceras deshayesi de Folin, 1869a: 27 , fig. 6 — Moore, 1972: 892; Abbott, 1974: 94; Gomes, 1999: 19, fig. 13; Lester, 2017: 27.

Meioceras moreleti de Folin, 1869a: 26 , fig. 5 — Kisch, 1959: 40; Moore, 1972: 892; Abbott, 1974: 94; Gomes, 1999: 21, fig. 18–19; Lester, 2017: 27.

Meioceras subinflexum de Folin, 1869b: 165 , pl. 23, fig. 8 — Abbott, 1974: 94; Gomes, 1999: 22, fig. 20; Lester, 2017: 27.

Meioceras undulosum de Folin, 1869a: 28 , fig. 8 — Kisch, 1959: 40; Moore, 1972: 892; Abbott, 1974: 94; Gomes, 1999: 22, fig. 21–22; Lester, 2017: 27.

Meioceras fischeri de Folin, 1870: 188 , pl. 26, fig. 3–4 — Kisch, 1959: 40; Moore, 1972: 892; Abbott, 1974: 94; Gomes, 1999: 24, fig. 26; Lester, 2017: 28.

Meioceras imiklis de Folin, 1870: 189 , pl. 26, fig. 5–6 — Kisch, 1959: 40; Moore, 1972: 892; Abbott, 1974: 94; Gomes, 1999: 24, fig. 27; Lester, 2017: 28.

Meioceras contractum de Folin, 1874: 213 , pl. 9, fig. 7 — Lester, 2017: 28.

Meioceras leoni Bérillon in de Folin, 1874: 251 , pl. 10, fig. 4 — Moore, 1972: 892; Abbott, 1974: 94; Gomes, 1999: 25, fig. 28–29.

Meioceras elongatum de Folin, 1881: 17 , fig. 9 — Vannozzi, 2019: 35, fig. 1–4.

Meioceras cingulatum Dall, 1892: 302 , pl. 16, fig. 6–7 — Moore, 1972: 892; Lester, 2017: 28.

Caecum (Meioceras) lermondi Dall, 1924: 7 — Lester, 2017: 28; Moore, 1972: 892; Gomes, 1999: 26, fig. 31.

Meioceras apanium Woodring, 1928: 351 , pl. 26, fig. 11–12 — Moore, 1972: 892.

Meioceras amblyoceras Woodring, 1959: 163 , pl. 31, fig. 1 — Moore, 1972: 892.

Original diagnosis — "Shell arcuated, thin, pellucid; surface white, shining, glabrous, with indistinct striae of growth; aperture very oblique, in diameter about two thirds that of the shell at its broadest part, which is at the middle. The shell is contracted at its posterior extremity. Thus, the inner outline is much shorter and less curved than the outer one." Stimpson (1851: 112).

Referred Specimens —LACMIP 43085.15 (1 specimen), LACMIP 43085.16 (5 specimens), LACMIP 43085.17 (6 specimens), LACMIP 43085.18 (94 specimens).

Occurrence — Known only from immediately above and below the Thylacodes beds near the base of section SC 3 in Super Creek, Riverside County, California .

Description —Protoconch and subadult stages not observed. Teleoconch average size for genus [Tol: 1.74–2.40 mm], tubular, strongly arched dorsal profile, mildly arched or bulbous ventral profile [Larc: 0.52–0.80 mm; Arc: 0.08–0.22 mm], narrow posterior end, inflated middle region and contracted apertural end, creamy white. Surface smooth except for occasional growth lines. Posterior [Dpe: 0.30–0.39 mm] with thin edge. Septum mucronate, slightly raised to recessed. Mucro with prominent rounded point, positioned along dorsal margin. Aperture [Da: 0.40–0.54 mm] simple, strongly oblique. Lip smooth, well defined.

Discussion — Meioceras nitidum is extremely variable in the degree of mid-teleoconch swelling. Slender forms can be difficult to distinguish from M. cornucopiae Carpenter, 1859 . However, the sides of M. cornucopiae are more parallel regardless of how pronounced the dorsal arch.

Figures

Figure 7A–C. Meioceras nitidum: A–B, LACMIP 43085.13, LACMIP 43085.14, profile views; C, LACMIP 43085.13, ventral view. Scale bar=500μm.