Shell small to large, gaping posteriorly, « inequivalve with left valve generally larger and extending slightly beyond right; shell usually with a weakly developed prismatic layer and a thick biphasis (lenticular plus sheet) nacre; periostracum often well developed and extending beyond shell edge especially in posterior region;internal resilium split by a well developed lithodesma; resilium possesses a dense array of long, aragonitic needles which lie in a plane normal to hinge and lithodesma… » – R. S. Prezant: “Taxonomic re-evaluation of the bivalve family Lyonsiidae”, The Nautilus vol.95, Sanibel 1981, p.61.
Lyonsia Gray, 1853:
« Shell elongate, inequilateral, posterior region from umbones usually longer and taperingto a truncated siphonal region; shell with siphonal gape; typically a dense cover of foreign material, usually sand, covering at least a portion of the shell’s exterior; shell radially ornamented with numerous, small truncated spines; periostracum thin with fine radial striations and adherent mucoid strands emanating from mantle edge; small but prominent lithodesma; prismatic shell layer thin and lenticular and sheet nacre well developed; […] isomyarian to slightly heteromyarian&hellup; » –R. S. Prezant: “Taxonomic re-evaluation of the bivalve family Lyonsiidae”, The Nautilus vol.95, Sanibel 1981, p.64.
