The biserial archipterygium consists of a segmented axial rod, bearing a praeaxial and a postaxial series of jointed rays.
Each limb then has a dorsal surface, a ventral surface, an anterior or pre-axial edge, and a posterior or postaxial edge.
The proximal part of the postaxial surface also bears a small outgrowth.
In the anterior limb the radius and the pollex are pre-axial, the ulna and the fifth finger are postaxial.
In the posterior limb the tibia and the hallux are pre-axial, the fibula and the fifth toe are postaxial.
Archipterygia too are found in the pectoral fins of the Ichthyotomi, but the postaxial radiale are much reduced.
The ulna or postaxial bone is somewhat expanded distally, but is not drawn out proximally into an olecranon process.
The anterior part of the pre-axial border and posterior part of the postaxial border bear slight outgrowths.
The ulna (fig. 48, A, 3) or postaxial bone is a curved bone rather larger than the radius.
postaxial post·ax·i·al (pōst-āk'sē-əl)
Posterior to the axis of the body or to the axis of a limb in the anatomical position.
Of or being the portion of a limb bud lying caudal to the axis of the limb.