Vol. 99 (2-3)
April 29, 1985
THE NAUTILUS 81
THE LAND SNAIL FAMILY HYDROCENIDAE IN VANUATU
(NEW HEBRIDES ISLANDS), AND COMMENTS ON
OTHER PACIFIC ISLAND SPECIES
Fred G. Thompson
Florida State Museum
University of Florida
Gainesville, FL 32611
and
Emilye L. Huck
P.O. Box 4413
Winter Park, FL 32793
ABSTRACT
Georissa obsoleta new species (Gastropoda, Prosobranchia, Archeogastropoda,
Hydrocenidae) is described from-Efate Island, Republic of Vanuatu. It is charac-
terized by its minute size, obsolete sculpture, channelled suture, and conical form.
It is the first record of Hydrocenidae from Vanuatu. Relationships with other
Pacific species are not clear. The Hawaiian Georissa neili Pilsbry, 1928 is
synonymizedwithG. cookei Pilsbry, 1928. G. kauaiensis Pilsbry, 1928 (Hawaiian
Islands). Chondrella striata Pease, 1871 (Cook Islands), and Cyclostoma
minutissima Sowerby, 1832 (Pitcaim Island) are based on juvenile specimens but
appear to be Georissa.
Key words: land snails, Hydrocenidae, Georissa obsoleta, Pacific Islands,
Vanuatu, New Hebrides Islands, Efate Island.
During June-August, 1984 the junior author
had the opportunity to visit the Republic of
Vanuatu, formerly called the New Hebrides
Islands. Significant collections of land snails
were made on Efate, Erromango, and Tanna.
The collections are particularly interesting
because of the large number of minute species
that were recovered from leaf-litter samples
gathered at many stations. A species of par-
ticular interest to us is described below. It is the
first record of the family Hydrocenidae from
Vanuatu.
Hydrocenid land snails are widely deployed on
Pacific islands, Japan, New Zealand, Australia,
the Indo-Australian archipellago, Southeast
Asia, Madagascar, Africa, and Mediterranean
Europe. They are poorly known because of their
minute size and cryptic behavior, although they
may be locally abundant (Thompson and Dance,
1983). Most species are confined to limestone
substrates and encrust their shells with lime or
mud. Live specimens appear more like minute
blobs of dirt than like coiled shells. Thus, it is not
surprising that this family was not reported in
the two principal papers on the New Hebrides
fauna (Solem, 1959, 1962). Because the new
species has very poorly developed shell
sculpture we name it:
Georissa obsoleta
Thompson and Huck, new species
Shell (Figs. 1-7): Minute, adults about 1.7-1.9
mm long, about 0.72-0.78 times as wide as high.
Color varying from yellow-gray to fulvous in
fresh shells. Conical with a wide rounded apex;
consisting of 3.3-3.7 whorls. Apex rounded, with
a large cap-shaped protoconch consisting of 1.3
whorls (Fig. 6). Protoconch sculptured with a
dense mesh of minute pits. Suture between
whorls of teleoconch very deeply impressed,
forming a channel around middle of shell (Fig.
5). Sculpture of teleoconch consisting of very
weakly developed spiral threads that form an
obsolete cancellate pattern where they cross
growth striations and threads (Fig. 2). Growth
threads tend to be enlarged above shoulder of
whorl and usually form weak denticles along
second and third whorls (Fig. 5). Aperture ovate
in shape, about as wide as or slightly wider than
high; about 0.39-0.48 times length of shell.
Parietal wall nearly straight, lying at an angle
of 30-32° to axis of shell (30° in holotype); plane
of aperture at 20-25° to shell axis (Fig. 3). Um-
bilical area with a wide shield that is indented
along outer edge (Fig. 7). Parietal septum ex-
tending into shell for % whorl, where upon the
earlier septum has been readsorbed (Fig. 4).
BioStor
