Axial costa

Axialcostae1
Image Credit: B. Van de Vijver
Scanning electron micrograph of the inside of a valve of Mastogloia smithii. Arrows indicate the axial costae on either side of the raphe.
Axialcostae2
Image Credit: B. Van de Vijver
Light micrographs of Mastogloia smithii, at two different focal depths. Each arrow indicates an axial costa.