Cambrian monoplacophoran molluscs (Class Helcionelloida)

Cambrian monoplacophoran molluscs (Class Helcionelloida) are morphologicall y diverse, but also show considerable interspecific variation. While usuall y small (1-2 tnm), they may achieve a size of 30 mm or more. Shell form ran ges from low limpers to high cones and these may be straight or curved; coi led shells may be bilaterally symmetrical, dextral or sinistral. This diver sity provides a fertile ground for speculation about early molluscan evolut ion and almost all extant classes of Mollusca have been recognised here. Ce rtainly, molluscs can be traced back to the late Vendian (late Precambrian) , but not all classes appeared during the Vendian-Cambrian transition. Smal l-sized late Vendian-Middle Cambrian molluscs (helcionelloids) show little resemblance to crown molluscan groups, including the living monoplacophoran s (tryblidiid Tergomya).