The shape of the bottom or ventral portion of the shell is a good indication of the three dreissenid species. For example, in the zebra mussel the ventral surface or bottom is distinctly flattened or, in some cases, somewhat concave to allow for greater stability when attaching to various substrates. In the quagga mussel and Mytilopsis, the bottom is more outwardly curved or convex.
Modified from Claudi and Mackie (1994) with permission.
For further information see Attachment to Bottom.