Description: A solid bivalve with 2 equal valves and a circular to oval outline. It reaches around 4.5 cm in length and is white with a very thick dark brown to black periostracum, which has ridges at the margin. Periostracum may be a light brown in juvenile animals. The outer surface of the shell is covered in fine concentric lines and these may form ridges near the umbones. 2 cardinal teeth in right valve, anterior one is broad while posterior is thin; 3 cardinal teeth in left valve, posterior of which is very small.

Habitat: Muddy sand or gravel.

Distribution: Seems to be a northern species with some scattered records from north and NE Britain and Ireland. Recorded from just outside Belfast Lough.

Similar Species: Could be confused with Tridonta elliptica.

Key Identification Features:

• 2 equal valves and circular to oval in shape.
• Very thick dark brown to black persiostracum.
• 2 cardinal teeth in right valve (anterior broad, posterior thin).
• 3 cardinal teeth in left valve (posterior very small).

Distribution Map from NBN: Tridonta borealis at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Tridonta borealis at iNaturalist World Species Observations database.

GBIF data for Tridonta borealis

WoRMS: Tridonta borealis at World Register of Marine Species. Accepted name: Tridonta borealis Schumacher, 1817. AphiaID: 152398.

Classification: Biota; Animalia; Mollusca; Bivalvia; Autobranchia; Heteroconchia; Archiheterodonta; Carditida; Crassatelloidea; Astartidae; Tridonta