The ossification of the olecranon has been examined in immature ulnae representative of several mammalian orders. It is found that, in primates, the olecranon develops from two constant epiphyses, viz. a superficial, scale-like traction epiphysis and a deeper, articular or pressure epiphysis, responsible for the ossification of the proximal portion of the articular trochlear fossa. In non-primates the olecranon manifests a single epiphysis, of the traction variety, remote from the trochlear fossa which is exclusively diaphyseal in constitution.