These primitive mollusks have a linear series of perforations for the exhalant mantle cavity currents.  Slits in the mantle and shell facilitate the excurrent water leaving the mantle cavity. This is a primitive solution to the sanitation problems caused by torsion (having the mantle cavity with the anus, nephridiopores, and gonopores located over the head).

 

       

 

Genetic diversity. The shells in any population of Umbonium are highly polymorphic.

 

       

 

Commonly called "button shells" because of their size and colors, these primitive mollusks inhabit sandy littoral habitats in the Indo-Pacific. Note the multispiral operculum.