Shell thin, cylindrical, smooth, translucent, often with coloured design. Molluscs of this group use the copious threads of their byssus to form nests for themselves.
Distribution : Cosmopolitan.
Remarks: The colourful species, A. arborescens, inhabits the West Indies.

Shell thin, frequently hyaline. Periostracum thin, adherent. Sculpture absent. Hinge plate weak. Ligament small, attached to slight resilial ridge. Members of this genus, which is known from the early Cretaceous, usually live colonially in a nest of byssal threads and adhering particles. Recorded to the early Cretaceous. The name is derived from the Latin amygdala, an almond; although almond is feminine, this derived generic name has been treated as neuter. Literature: J. F. Allen (1955), B. R. Wilson and Hodgkin (1967).