« Shell fragile, very inequilateral, somewhat flattened, the outline tending to be subquadrate, inequivalve. Prosogyrate beaks, rather acute and protruding. Posterior margin short, curved and, at about half its length, slightly undulate. Anterior margin depressed, rather quadrate. Ventral margin rounded. Dorsal margin straight, with a small prominence on the right valve, the corresponding prominence on the left valve present along its entire length. Glossy and semitransparent, sculpture of very fine, rather irregular and concentric growth lines, much more evident towards the region of the umbo. A very small pseudocardinal tooth on right valve. Lunule occupies about one half of the posterior side, is deep and marked by two distinct folds. Ligament internal, opisthodetic along the dorsal posterior margin, inconspicuous, occupying one third of the lunule. Muscle and pallial scars not evident in the dead specimen but slightly distinct in the living specimen. Colour white-yellowish. » – F. Carroza: “Thyasira alleni n. sp.”, Bollettino malacologico vol.17 suppl., Milano 1981, p.224.