Table 2 Main morphological differences between Mulinia lateralis, Mactra stultorum and Spisula subtruncata

Shell outline

triangular

oval

triangular

Radial ridge along posterior end of valves

distinct

absent

absent

Ligament

exclusively internal

external ligament present

external ligament present

Accessory lamella (between chondrophore and two fused cardinal teeth)

well developed

very weak

very weak

Shell colour

whitish to cream with thin, yellowish periostracum

creamy white with purple hue around umbos and brown radiating rays

dirty white with brown periostracum

Shell surface

smooth with very fine concentric growth lines

smooth

distinct concentric grooves along dorsal line on either side of umbos

Shell convexity

distinctly convex

valves moderately convex

variable, with distinctly convex and less convex forms (van Urk 1959)

Cardinal area between beaks

broad in large specimens

narrow in large specimens

narrow in large specimens