Theoretical work, laboratory studies, and field observations indicate that the oscillatory boundary layers generated by the tidal wave differ fundamentally in dynamics and kinematic structure from the unidirectional boundary layers of rivers. Unique to the former are mass-transport currents attributable to: (1) the wave motion itself, and (2) bed curvature in the presence of the oscillatory flow. The implication of this difference for bed-material transport is that the larger flow-transverse bedforms of shallow-water environments are divisible hydraulically between two major classes: (A) those related to tidal conditions, under which the fluid reverses in direction of flow with each reversal of the tide, permitting the initiation and maintenance of bed features by the spatially reversing, curvature-related mass transport, and (B) those related strictly to rivers and river-like flows, in which the fluid motion is unidirectional, and therefore the only mechanisms available for bedform initiation and maintenance are those creating a finite spatial lag between the transport rate and the bed waviness. Forms of Class B are best called dunes and bars, and only those attributable to Class A should be termed sand waves. The latter, restricted to oscillatory boundary layers of tidal origin, apparently correspond to the very much smaller; but also commonly symmetrical, ripple marks produced in wind-wave oscillatory boundary layers.