Protoconch morphology and peculiarities of the early ontogeny of the Cambrian helcionelloid mollusks

Morphology of the initial parts of shells of the ancient Cambrian mollusks is studied. New original data on size of the embryonic shell of the Cambrian gastropods are analyzed. It is supposed that mollusks had larvae of both direct (benthic) and planktic lecitotrophic types of development even as early as the Cambrian. The wide geographic distribution of some of the Cambrian mollusk species in the distant paleobasins of the world is a convincing evidence for the existence of planktic stage of development in the mollusk ontogeny of that period.