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Taxon profile

family

Rissoellidae M. E. Gray, 1850

kingdom Animalia - animals »  phylum Mollusca - mollusks »  class Gastropoda - gastropods »  order Allogastropoda »  superfamily Rissoelloidea

Description

The family Rissoellidae M.E. Gray, 1850 is made up of species with the shell small, oval-conoid, thin, smooth, transparent-vitreous, often iridescent with the spire more or less raised, with convex whorls, the last of which is rather swollen, with an obtuse apex, a thin sharp peristoma, a narrow umbilicus, sometimes only a slit. From the above, clearly distinctive features from many microspecies of the family Rissoidae do not appear. But there are both morphological and anatomical characteristics which clearly differentiate the species of the family Rissoellidae from those of the family Rissoidae: above all, the presence of 4 cephalic tentacles, two real tentacles and two protuberances, in between the tentacles, and of the same length.
Then the shape of the operculum, semicircular with acuminate apophysis and having, on the inside, in the centre, a sort of rudimentary rostrum (more or less pronounced). But, a notable distinction, the Rissoellidae show some Prosobranch characteristics (protoconch not heterostrophic, a simple foot, type of reproductive system, eggs in capsules) together with characteristics atypical to that subclass (2 pairs of tentacles, eyes placed in the centre of the tentacles, renal organ invading the top of the mantle, conformation of the operculum, etc), so this family has sometimes been considered to belong to the Prosobranchs - Thiele (1929), Fretter (1956), Fretter & Graham (1962), Nordsieck (1972), Golikov & Star-obogatov (1975) and sometimes to the Opisthobranchs -Fretter & Graham (1954).
Fasulo, G. 1989. Molluscs of the Gulf of Naples.

Interchangeable taxa

The following key is proposed for the Mediterranean species of the family Rissoellidae, to be applied to adult specimens in good condition of freshness:
1. a Shell with rather elongated spire (turriculated) R. diaphana
b Not turriculated shell (globose ovate) (2)
2. a Shell with rather squat spire, valvatiform, umbilicus evident, deep and round R.
globularis
b Ovate conical shell, rissoiform (3)
3. a Body whorl very globose, umbilical slit narrow, pink-opaline colouring (also in worn
specimens) ... R. opalina
b Body whorl less globose, spire slightly higher, umbilical slit more open, showing the
internal "naticiform" structure, whitish colouring, vitreous ... R. inflata
Fasulo, G. 1989. Molluscs of the Gulf of Naples.
Author: Jan Delsing

Included taxa

Number of records: 2

genus Cythnia Carpenter, 1864
genus Rissoella Gray, 1847

Links and literature

EN The Interim Register of Marine and Nonmarine Genera [110845]

Rees, T. (compiler): The Interim Register of Marine and Nonmarine Genera [http://www.irmng.org] [as Rissoellidae Gray, 1850]
Data retrieved on: 11 February 2020
EN Australian Faunal Directory [18d10410-3b4a-4126-8808-abf8ab5b626e]

ABRS (2009-2019): Australian Faunal Directory [https://biodiversity.org.au/afd/home], Australian Biological Resources Study, Canberra [as Rissoellidae Gray, 1850]
Data retrieved on: 13 February 2015

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