A bivalve mollusc (Thracia convexa)

Distribution data supplied by the Ocean Biodiversity Information System (OBIS). To interrogate UK data visit the NBN Atlas.Map Help



This bivalve can be up to 7 cm long and has an inflated triangular shape. The shell is brittle and white in colour, rarely cream. The outer thin layer of shell (periostracum) is greenish-yellow underneath which the shell has a granulated surface. The outer shell has irregular concentric lines and undulations and a smooth ridge runs from the beak (umbones), which is almost exactly midline, to the posterior ventral corner.

Recorded distribution in Britain and Ireland

Widely distributed around the British Isles except off the eastern coast of England and the northwest coast of Ireland.

Global distribution

From Norway south to the Mediterranean.


The deep burrowing bivalve lives in sand and mud offshore.

Depth range


Identifying features

  • Triangular, white bivalve up to 7 cm long.
  • Right valve slightly larger and more convex than left valve.
  • Irregular concentric lines and undulations on shell.
  • Granulated shell surface.
  • Greenish-yellow periostracum.
  • Smooth ridge from umbones to posterior ventral corner.
  • Beak of left valve sitting in a hole in the umbones of the right.
  • No teeth present.

Additional information

Found in sand and mud offshore but never in large numbers.

Listed by

- none -


  1. Bruce, J.R., Colman, J.S. & Jones, N.S., 1963. Marine fauna of the Isle of Man. Liverpool: Liverpool University Press.

  2. Christensen, J.M., 1980. Seashells. Bivalves of the British and Northern European Seas. Revised and adapted by Peter S. Dance. Hamondsworth: Penguin Books Ltd.

  3. Crothers, J.H. (ed.), 1966. Dale Fort Marine Fauna. London: Field Studies Council.

  4. Foster-Smith, J. (ed.), 2000. The marine fauna and flora of the Cullercoats District. Marine species records for the North East Coast of England. Sunderland: Penshaw Press, for the Dove Marine Laboratory, University of Newcastle upon Tyne.

  5. Howson, C.M. & Picton, B.E., 1997. The species directory of the marine fauna and flora of the British Isles and surrounding seas. Belfast: Ulster Museum. [Ulster Museum publication, no. 276.]

  6. Picton, B.E. & Costello, M.J., 1998. BioMar biotope viewer: a guide to marine habitats, fauna and flora of Britain and Ireland. [CD-ROM] Environmental Sciences Unit, Trinity College, Dublin.

  7. Seaward, D.R., 1982. Sea area atlas of the marine molluscs of Britain and Ireland. Peterborough: Nature Conservancy Council.

  8. Seaward, D.R., 1990. Distribution of marine molluscs of north west Europe. Peterborough: Nature Conservancy Council.

  9. Seaward, D.R., 1993. Additions and amendments to the Distribution of the marine Molluscs of north west Europe. , Joint Nature Conservation Committee, Peterborough. [JNCC Report no. 165].

  10. Tebble, N., 1976. British Bivalve Seashells. A Handbook for Identification, 2nd ed. Edinburgh: British Museum (Natural History), Her Majesty's Stationary Office.


  1. Centre for Environmental Data and Recording, 2018. Ulster Museum Marine Surveys of Northern Ireland Coastal Waters. Occurrence dataset https://www.nmni.com/CEDaR/CEDaR-Centre-for-Environmental-Data-and-Recording.aspx accessed via NBNAtlas.org on 2018-09-25.

  2. Conchological Society of Great Britain & Ireland, 2018. Mollusc (marine) data for Great Britain and Ireland - restricted access. Occurrence dataset: https://doi.org/10.15468/4bsawx accessed via GBIF.org on 2018-09-25.

  3. Conchological Society of Great Britain & Ireland, 2018. Mollusc (marine) records for Great Britain and Ireland. Occurrence dataset: https://doi.org/10.15468/aurwcz accessed via GBIF.org on 2018-09-25.

  4. Fenwick, 2018. Aphotomarine. Occurrence dataset http://www.aphotomarine.com/index.html Accessed via NBNAtlas.org on 2018-10-01

  5. NBN (National Biodiversity Network) Atlas. Available from: https://www.nbnatlas.org.

  6. OBIS (Ocean Biodiversity Information System),  2024. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. www.iobis.org. Accessed: 2024-07-14


This review can be cited as:

Neish, A.H. 2005. Thracia convexa A bivalve mollusc. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 14-07-2024]. Available from: https://www.marlin.ac.uk/species/detail/1896

Last Updated: 17/10/2005