1. The full-grown veligers of Mytilus crassitesta (text-fig. 1, pl.3, figs. 2, 11) were identified by means of tracing back the prodissoconchs on the umbonal portion of the young shells (pl.3, fig.1). 2. The full-grown veliger attains a length of between 0.28-0.32mm, when the velum becomes reduced, the pelagic veliger stage ends, and it enters into the completely benthic life. 3. Through the more rapid and broader growth of the ventroposterior portion of the shell, the dorso-anterior portion takes the form of a pointed angle, resulting in a change in the directions of the greatest longitudinal and vertical axes of the shell. The adult form is thus attained, accompanied by the appearance of the reddish purple coloration. 4. Teeth on the dorso-posterior rim gradually become reduced during growth and disappear in the adult, while the reduction of the dorso-anterior teeth is not complete, 2 or 3 teeth always remain on the inside of the umbo through life (text-fig. 2). This denticulation is one of the most important characteristics by which the young shell of this species is distinguished from those of the related species.

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