Amussiidae.-Shell orbicular, smooth externally with radiating costae internally.
Carditidae.-Shell thick, with radiating costae; foot carinated, often byssiferous.
Shell equivalve, with radiating costae and external ligament.
- Corallum of Caryophyllia; semi-diagrammatic. th, Theca; c, costae; sp, septa; p, palus; col, columella.
(5) The costae, longitudinal ribs or rows of spines on the outer surface of the theca.
True costae always correspond to the septa, and are in fact the peripheral edges of the latter.