Trigoniidae

Although they were abundant in the Mesozoic era, they are today represented by only one living genus, Neotrigonia, which inhabits waters off the coast of southern Australia.

Etheldred Benett added several Upper Jurassic species, although her work was not primarily recognised due to the academic status of women at that time.

The major worker on the Trigoniidae in the nineteenth century was John Lycett, a physician from Gloucestershire who published a text entitled A Monograph of British Fossil Trigoniae.

Today, knowledge is sufficient to divide the family into five Subfamilies (see below), which together contain more than sixteen genera, the most abundant being Trigonia, Myophorella, Laevitrigonia, and Orthotrigonia.

The hinge structure is amongst the most complex of all bivalves, namely that the teeth are numerous and ridge-like with strong transverse striations.

In Britain, examples are numerous in the Upper Jurassic rocks of the Dorset coast, particularly around the village of Osmington Mills.

The genus Trigonia is the most readily identifiable member of the family, having a series of strong ribs or costae along the anterior part of the shell exterior.

The first European examples (Trigonia costata Parkinson) turn up in the Lower Jurassic (Toarcian) of Sherborne, Dorset and Gundershofen, Switzerland.

a) Neotrigonia, showing radial ornament across the exterior of the shell; b) Eotrigonia, the likely ancestor of Neotrigonia; c) Juvenile example of Neotrigonia, showing ornament similar to Eotrigonia. This ornament is lost in later stages of growth.
a) Neotrigonia , showing radial ornament across the exterior of the shell; b) Eotrigonia , the likely ancestor of Neotrigonia ; c) Juvenile example of Neotrigonia , showing ornament similar to Eotrigonia . This ornament is lost in later stages of growth.
Internal mold of Laevitrigonia gibbosa from the Portland Limestone ( Upper Jurassic ), Isle of Portland , Dorset, England
The trigoniid hinge, showing interior view of the left valve (A) and right valve (B) of Trigonia . The notation 2, 3a, 3b, 4a, 4b and 5a refer to the numbers assigned to individual teeth. "aa" and "pa" are the anterior and posterior muscle scars and "ppr" is the scar left by the posterior pedal retractor muscle.
Diagram of Trigonia costata James Parkinson , showing main morphological features of the shell exterior; a) Anterior ; p) Posterior ; d) Dorsal ; v) Ventral ; F) Flank; A) Area; c) Costae ; mc) Marginal Carina . Trigonia costata ranges from the Lower Jurassic ( Toarcian ) to Middle Jurassic ( Callovian ).