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Bollettino Malacologico, Voi. 32 N° 5-8, pp 145-150, 1996 Società Italiana di Malacologia
Stampato in Italia, Roma 15 novembre 1997 Lavoro accettato il 13 ottobre 1997

A NEW POLYPLACOPHORAN SUBGENUS FOR THE NORTHEAST ATLANTIC
(ISCHNOCHITONIDAE: CALLISTOPLACINAE)

BRUNO DELL’ANGELO & MARCO OLIVERIO

Key Words: Allerychiton subg. nov., Callistochiton (A.) pachylasmae, Northeast Atlantic, Mediterranean, Callistochiton,
biogeography.
Abstract: Allerychiton subg. nov. is here introduced for Callistochiton pachylasmae (Monterosato, 1879), on the base of
peculiarities of the sculpture of the intermediate and end valves. Its isolated position within the genus Callistochiton
Dali, 1879 [ex Carpenter MS] is supported by the available geographic data on the species of the genus.
Riassunto: Allerychiton subg. nov. viene qui proposto per Callistochiton pachylasmae (Monterosato, 1879), sulla base
di peculiarità nella scultura delle piastre intermedie (una sola costa radiale), e delle piastre terminali (con scultura diffe-
rente tra le due piastre). L’identificazione di una linea filetica distinta nell’ambito del genere Callistochiton Dall, 1879
[ex Carpenter MS], è supportata dall’isolamento geografico di pachylasmae sulla base dei dati disponibili sulla distribu-
zione delle altre specie del genere.
BRUNO DELL ANGELO, Via Mugellese 66D. 1-50047 Prato, Italia.
MARCO OLIVERIO, Dipartimento di Biologia Animale e dell’Uomo, “La Sapienza” Rome 1 University. Viale dell’Uni-
versità 32. 1-00185 Roma, Italy. Present address: Dipartimento di Biologia, Rome 3 University. Viale Marconi 446,
100146 Roma, Italy. E-mail : moliverio@axrma.uniromal.it

Introduction
The genus Callistochiton Dali, 1879 (type species C. palmulatus Dali, 1879) is presently included
(Kaas & Van Belle, 1994) in the subfamily Callistoplacinae (family Ischnochitonidae) together with
Callistoplax, Calloplax, Ceratozona and Ischnoplax, mainly differentiated on perinotum characters. It is
characterised by the tegmentum conspicuously sculptured and the end valves and lateral areas of inter-
mediate valves with strong radial ribs. Characteristically, the lateral areas are bicostate on either side
(Ferreira, 1979). Callistochiton presently includes 36 validly recognised species according to Kaas &
Van Belle (1994) all from warm temperate and tropical waters (Tab. 1). Many taxa are known only by
the holotype or the type series, hampering a good knowledge of the actual range of each species.
The sole NE Atlantic and Mediterranean species is C. pachylasmae (Monterosato, 1879), with
only three specimens so far known (see Pizzini & Oliverio, 1993: the specimen from the Atlantic coast
of Morocco (Kaas, 1991) was unfortunately lost). Two loose valves have been recently found (see
below), widening the known range.
The species is very peculiar in the ambit of the genus Callistochiton because of the sculpture of the
valves. The intermediate valves bear only a single elevated rib on each lateral area. The posterior end
valve has a sculpture different from that of the head valve, a feature very rare within the Polyplacopho-
ra. These two features characterise C. pachylasmae within the genus, and the biogeographical data point
out the isolation of this species in the world pattern. It is the sole East Atlantic species so far known,
while the genus is well represented throughout the Indo-Pacific and the Caribbean-Brazilian tropical
and subtropical areas (see Tab. 1). A Callistochiton species was present in the Mediterranean Pliocene
of Italy (Dell’Angelo & Vazzana, unpublished data): only the first plate is known of the fossil speci-
men, the sculpture is similar to that of pachylasmae and its belonging to the same lineage would witness
for an established history of the pachylasmae-lineage. We consider C. pachylasmae as representing a
distinct lineage within the genus, and propose for it a subgenus on its own, dedicated to the author of

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