Type species Basilissa bombax Cotton & Godfrey, 1938 (Recent; South Australia).
Description. Shell 2.30-3.50 mm high at maturity, about as high as broad, rather thick, umbilicate, nacreous within.
Protoconch of 1 convex, finely granulate whorl.
Teleoconch of 4.75-6.00 whorls at maturity; spire whorls shallowly convex or flat; periphery gently angulate; base rather gently contracted, shallowly convex. Primary sculpture consisting of narrow, crisp spiral cords and collabral axial riblets. Spiral cords multiplying by intercalation to number 5-7 on spire and 9-12 on base; abapical 2 spire spirals strongest, peripheral; abapical peripheral spiral with summit covered by succeeding whorls. Spire spirals sharply nodular at intersections with axials. Basal spirals more weakly nodular; innermost spiral with strong, rounded nodules that enter umbilicus as evanescent radial pleats. Axial riblets crisp on spire, weaker or obsolete on base. Secondary sculpture consisting of minute, crisp granules in interspaces of primary sculpture. Aperture subrhomboidal. Outer lip thin at rim, thickened immediately within against abapical side of posterior notch and around basal notch; posterior and basal notches shallow; depth of posterior notch 7.4-12.5% of shell diameter; no peripheral notch. Inner lip thick, gently reflected into umbilicus at rim, gently flexed at base to form a small, rounded tooth that projects into aperture.
Calliobasis is evidently closely related to Basilissa (s.s.) and Ancistrobasis, species of which have essentially similar sculpture and much the same outer lip profile. Calliobasis species differ in having a stouter, more compact shell, fewer spire spirals, and a narrowly biangulate periphery (cf. mono-angulate). This compact genus contains the only seguenziids known to have a colour pattern. Apart from C. bombax, the genus contains the New South Wales species Astek bilix Hedley (1905, p. 48, fig. 13), the new species described below, and perhaps Basilissa (Ancistrobasis) compsa Melvill (1904, p. 160, pi. 10 fig. 4), from the Gulf of Oman, and the Australian Tertiary species Basilissa cossmanni Tate (1894, p. 185, pl. 11 fig. 8). 'Seguenzia9 radialis Tate (1890, p. 192, pi. 9 fig. 6, 6a) resembles the taxa included here in general facies, but differs in having a strong fold at the base of the inner lip; while undoubtedly a seguenziid, its affinities are unknown.