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the species who first noticed and described the differences in the end valve, a character overlooked for
more than one century.

Systematics

Family ISCHNOCHITONIDAE Dali, 1889
Subfamily CALLISTOPLACINAE Pilsbry, 1893
Genus Callistochiton Dali, 1879 [ex Carpenter MS]
Subgenus Allerychiton subg. nov.

Diagnosis. Tegmentum with rough sculpture, uniformly covered with large quincunxially set tubercles;
cephalic valve with 7 large radial ribs; lateral areas of intermediate valves with a single elevated radial
rib; posterior valve with one (smaller) radial rib on either side of the muero. Perinotum (girdle) wide,
covered on the external surface with imbricating and deeply grooved scales, bearing spherules on the
upper margin; ventral surface with radiating rectangular scales.
Type species: Callistochiton pachylasmae (Monterosato, 1879).

Callistochiton (Allerychiton pachylasmae (Monterosato, 1879)
)
Chiton Pachylasmae - Tiberi, 1878: 150 ( nomen nudum)
Chiton Pachylasmae Seg. ms - Monterosato, 1878: 77 {nomen nudum)
Chiton Pachylasmae - Monterosato, 1879: 24
Chiton pachylasmae (Seg. Msc.) Monter. - Cams, 1893: 180
Chiton pachylasmae (Seguenza MS) Monterosato - Pilsbry, 1894: 95
Callistochiton pachylasmae (Monterosato) - Sabelli, 1971: 561
Callistochiton pachilasmae (Monterosato, 1879) - Sabelli, 1974: 76
Callistochiton pachilasmae - Ferreira, 1979: 463
Callistochiton pachylasmae (Seguenza in Monterosato, 1878) - Piani, 1980: 176
Callistochiton pachylasmae (Di Monterosato, 1879) - Van Belle, 1981: 151
Callistochiton pachylasmae (di Monterosato, 1879) -Van Belle, 1984: 136
Callistochiton pachylasmae - Bedulli et al., 1985: 60
Callistochiton pachylasmae (Seguenza in Monterosato, 1878) -Gaglini, 1985: X, plt..2
Callistochiton pachylasmae (Seguenza in Monterosato, 1878) -Bruschi et al., 1985: 44
Callistochiton pachylasmae (Monterosato, 1878 ex Seguenza G. ms.) - Sabelli et al., 1990: 7, 1 16
Callistochiton pachylasmae (Di Monterosato) - Kaas, 1991: 93
Callistochiton pachylasmae (Seguenza in Monterosato, 1879) - Poppe & Goto, 1991: 59
Callistochiton pachylasmae (Monterosato, 1878 ex Seguenza G. ms.) - Sabelli et al., 1992: 352
Callistochiton pachylasmae (Monterosato, 1879) - Giovine & Dell’Angelo, 1993: 163
Callistochiton pachylasmae (Monterosato, ex Seguenza G. MS) - Pizzini & Oliverio, 1993: 41, Fig. 1
Callistochiton pachylasmae (Seguenza MS, di Monterosato) - Kaas & Van Belle, 1994: 123, Fig. 48, map 34
Callistochiton pachylasmae (Monterosato, 1878 ex Seguenza ms.) - Nicolay & Angioy, 1994: 29, Fig. 1
Ischnochiton pachylasmae (Monterosato) - Arduino et al., 1995: 10

Material
Holotype, from the Messina Strait (Sicily, Italy), in ZMR 5.5 mm length and 3 mm width (Figs 1-3).
One specimen from Pantelleria Is. (UR92-7#2 stn., 36°49’24” N - 012°00’53” E -53.4 m: Pizzini & Oli-
verio, 1993) 3 mm length and 1.2 mm width (Figs 8-13). One head valve from Punta Longa “Secca
Galera” (Favignana Is.), -33 m (coll. Quadri). One intermediate valve from Ormos Panagias, -35/40 m
Sithonia Is., Greece, (A. Lugli leg. 8/82, - coll. Dell’Angelo No PI 85/1: Figs 4-7).

Bollettino Malacologico 32 - 146-