| Abstract
| - Recent studies on the anatomy and sperm morphology of the Conirostridae, a marine group closely related to the northern-hemisphere freshwater family Valvatidae, and the Orbitestellidae, a world-wide family of minute, discoidal marine gastropods, show them to be primitive Heterobranchia. The characters possessed by these gastropods are used to reassess what early heterobranchs may have been like. It is suggested that heterobranchs and caenogastropods either evolved from a common pre-caenogastropod ancestor or from two separate groups having the archaeo-gastropod grade of organisation.
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