Early shell development at the larval and early postlarval stages of a boring bivalve Zirfaea subconstricta is described using scanning electron microscopy, with discussion of its functional significance for achieving boring behavior. In Z. subconstricta, the boring function performed by the shell morphology drastically develops in a short time after the beginning of metamorphosis. This species shares common characteristics with other pholads regarding the early ontogenetic appearance of the boring morphology. These characteristics provide much information of primary shell morphology as basal condition of boring function, which is usually modified during the later growth.