Trivia monacha

10mm to 12 mm high (longest dimension of shell) by about 8mm wide.Typical ovoid cowrie shape. Solidly built with about twenty strong transverse ridges and intervening grooves. Spire hidden from view by the enveloping final whorl [image8]; a slight bump on the posterior end may indicate its position.Ventral surface white with a slit aperture.On the rest of the shell the ridges are pinkish white with pink to dark pink intervening grooves, and there are three dark marks of varying intensity on the dorsum [3].

White flimsy shell [13]. Up to 12 mm high (longest dimension of shell). Last stage juveniles [9] can be larger than early adults. Under 7mm high the juvenile shell [10] has a typical gastropod aperture and spire, and lacks any sign of ridges. At 10mm high [11] the spire is very small in proportion to the final whorl, the aperture is a slit, and white ridges are developing on the ventral surface. Shape more ovoid. At 12 mm high [12] the spire is even smaller in proportion to the final whorl, the aperture is a slit, and ridges are appearing on both ventral and dorsal surfaces. During transformation to adult, the spire is completely enveloped by the final whorl.

The mantle is drawn out at the front of the animal into a cylindrical respiratory siphon with a slit down its anterior face. Head, smooth tentacles and siphon are orange in adults [1], with many yellow longitudinal lines on the dorsal surface of the foot [2], and yellow spots on the tentacles and siphon. The mantle, can envelope the whole shell [5], or be completely withdrawn [3]. It is greyish white, heavily marked with dark, often purplish, patches aligned with the shell grooves. Small yellow tubercles are scattered sparsely over its surface. The free edges of the mantle are orange.