DIAGNOSIS: A moderately sized, ovately shaped Nucula, smooth or partly sculptured with concentric ribs, with a distinct radial structure in the median section of the valve (appearing as radial "lines" or striae) and a correspondingly denticulated inner ventral margin, and with a short but oblique chondrophore. From its nearest relative, Nucula beachportensis Verco, it differs in being smaller, with a usually less truncated, relatively longer posterior area and a relatively shorter posterior margin, and a less oblique and more projecting chondrophore in adult specimens. The two species seem to differ also in habitat, N. pusilla preferring rather shallow waters while N. beachportensis is restricted to deeper waters.
DESCRIPTION: Valve moderately solid, quadrately ovate in outline, not very small. Dorsal margin slightly curved. Posterodorsal and posteroventral angulations only sharp in young specimens and usually more rounded in adult ones. Posterior side somewhat truncated, weakly rounded. Anterodorsal curve gradual, anterior side weakly rounded and with a broad curve extending to the evenly rounded ventral margin. Embryonic shell with a granular appearance on median exterior, and a smooth margin. Interdissoconch smooth but for growth lines. Rest of valve with concentric ribs on anterior and posterior areas, sometimes covering the median area also. Usually a few well developed concentric ribs near ventral margin. Beyond interdissoconch shell distinctly radially structured with narrow, prismatic elements (see Taylor, Kennedy and Hall, 1969). Periostracum greenish, sometimes brownish. Shell white. Hinge line arched, chondrophore short, pointing forwards. In lectotype (length 2.60 mm) 10 anterior and 4 posterior, V-shaped secondary teeth. In younger shells some small primary tooth-like projections may be present beneath the beak (Bergmans, 1968). Adductor muscle scars and pallial line usually indistinct. Inner margin finely denticulated by radial structure. The sculpture on the exterior of adult shells of Nucula pusilla can be divided into four zones:
1. The microscopically pitted surface of the median part of the embryonic shell.
2. The rather narrow smooth marginal zone of the embryonic shell.
3. The comparatively large interdissoconch, which is smooth but for some fine growth lines.
4. The valve beyond the interdissoconch, with concentric ribs covering anterior and posterior areas and not infrequently also the median area.

Along the coasts of southern New South Wales, Victoria, Tasmania, South Australia and southern and western Western Australia