Shell small, cylindrical, rounded anteriorly, truncate posteriorly. Colour dull white. Sculpture: tine radial threads which enlarge and curve in on the vertex. Aperture anteriorly pyriform, medially narrow and constricted, posteriorly rising above the last whorl and looped over towards the axis, a small oblique plait on the columella. Spire deeply sunk, the summits of previous whorls visible, sculptured by transverse riblets descending to a papillary apex. Length, 3; breadth, 1,5 mm.
Hab. —Middle Harbour, Sydney (self). This is like R. amphizosta from North Queensland, but lacks spiral sculpture, is not as compressed at the waist, has the aperture carried up higher, and the spire more deeply sunk. From Portsea, Victoria, Messrs. Gatliff & Gabriel record a variation of R. amphizosta, which may possibly be this. Cylichna atkinsoni Ten.-Woods, which also occurs in Sydney Harbour, is larger than R. iredaleana, more inflated medially, more contracted posteriorly, with a narrow vertical pore.
Named in honour of Mr. Tom Iredale, and in remembrance of the excellent critical work that he has done on the Australian mollusca.