Discartemon discus (L. Pfeiffer 1851)

Shell. Shell flattened, white and translucent; whorls 6–6½; spire flattened with distinct suture. Shell surface glossy, smooth with growth lines and varices present. Embryonic shell large, about 2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl angular, regularly expanded; umbilicus very wide, shallow and showing all preceding whorls. Aperture semi-ovate; peristome discontinuous, thickened, expanded and reflected; apertural dentition with only one parietal lamella (Fig. 4A–C). Radula. Each row consists of 61–67 teeth with formula (30-33)-1-(30-33). Central tooth very small and triangular with pointed cusp. Lateral and marginal teeth undifferentiated, unicuspid and lanceolate. Latero-marginal teeth gradually reduce in size, with outermost teeth smaller and shorter than inner teeth (Fig. 22A). Genital organs. Atrium (at) short; penis (p) long and slender. Penial sheath (ps) thin, extending about half to third-fourths of penis length; penial sheath retractor muscle (psr) very thin, originating at atrium and inserting at distal end of penial sheath (Fig. 11A). Vas deferens (vd) passes through a very short part of penial sheath before entering into penis distally (Fig. 11B). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction. Vagina (v) long, cylindrical, about two thirds of penis length. Gametolytic duct (gd) a long tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) short; oviduct (ov) enlarged and folded. Prostate gland inconspicuous and bound to oviduct. Talon (ta) small. Hermaphroditic duct (hd) bearing long seminal vesicle (sv) about one and half times longer than the length from talon to branching point of seminal vesicle (Fig. 11A). Pallial system. Excretory system typically sigmurethran and without mantle gland. Heart (h, auricles and ventricle) located left of kidney (on right in Fig. 11C). Pulmonary cavity approximately three times longer than broad. Pulmonary vein (puv) and venation on lung roof distinct and well developed. Kidney (k) very short, located at posterior of pulmonary cavity. Ureter (ur) sigmoid, closed tube arising from apex of kidney, extending along right side of kidney, recurving adjacent to rectum (rt). Anus (a) adjacent to pneumostome (pn) on mantle collar. Shell measurements of Discartemon spp (Discartemon discus-group). Specimen collections and catalogue numbers indicated in parentheses. Species and locality and CUMZ nos No. of specimens Ranges, mean ± S.D. in mm of: Number of whorls Shell height Shell width H/W ratio Discartemon discus (Pfeiffer, 1853) Da Nang, Vietnam: (6001, 6257) 45 4.2–6.7 5.0±0.43 11.78–14.26 12.9±0.59 0.3–0.5 0.4±0.03 6–6½ Discartemon nummus (Laidlaw, 1929) Khao Ok Thalu, Phatthalung: (3594) 24 2.4–3.3 2.8±0.21 6.1–7.2 6.5±0.27 0.4–0.5 0.4±0.03 5½ Discartemon khaosokensis Panha & Burch, 1998 Khao Sok N. P., Suratthani: (6242, 6243) 5 3.4–4.0 3.6±0.26 11.2–12.4 11.8±0.47 0.3–0.3 0.3±0.02 5½–5¾ Discartemon discadentus sp. n. Wat Tam Yai, Suratthani: (6209, 6244, 6258) 16 3.9–5.8 5.0±0.49 10.1–15.4 11.9±0.32 0.4–0.5 0.4±0.04 6 Wat Tam Wararam, Suratthani: (3571) 15 5.5–7.0 6.3±0.33 12.4–13.6 13.1±0.32 0.4–0.5 0.4±0.02 6 Discartemon discamaximus sp. n. Tam Namphud, Phangnga: (6005, 6245) 5 4.7–5.0 4.9±0.10 12.4–13.6 14.3±0.49 0.3–0.4 0.3±0.02 7 Tam Kobe, Phangnga: (3669, 6197) 17 4.4–5.6 4.9±0.38 10.8–13.7 12.3±0.76 0.4–0.4 0.4±0.02 7 Discartemon circulus sp. n. Tam Phannara, Nakhon Si Thammarat: (3665, 6246, 6262) 23 3.0–4.5 3.7±0.32 7.7–9.5 8.6±0.45 0.4–0.5 0.4±0.03 5½–6 Discartemon deprima sp. n. Khao Hup Ta Hae, Prathiew, Chumphon: (3573, 6247) 7 2.5–3.4 2.9±0.30 8.2–10.3 9.1±0.72 0.3–0.3 0.3±0.02 5–5½ Ban Tam Thong, Prathiew, Chumphon: (6259) 5 3.1–3.8 3.5±0.27 9.5–11.2 10.1±0.68 0.3–0.3 0.3±0.01 5–5½ Discartemon expandus sp. n. Klong Hoy, Suratthani: (3664, 6248) 16 3.6–4.3 4.0±0.30 8.2–10.9 9.9±0.84 0.4–0.5 0.4±0.03 5½–6