Measurement of length and height in Asian clams and dreissenids is complicated by the fact that there is a distinct change in body orientation of dreissenids as they mature. The original “anterior” portion of the shell in immatures slows in growth and subsequently forms the “ventral” surface of the adult. Thus a major change occurs in what is conceived as the length and the height.
In this regard, for purposes of immature identification, conventional measurements are used instead of purely morphological measurements.
Therefore, in Asian clams and clam-shaped dreissenids, height is defined as the distance from umbone, or middle of the hinge line, to the opposing valve margin, and length is measured perpendicular from the height line.
In contrast, in mussel-shaped plantigrade and juvenile dreissenids, length is defined as the distance from the umbo to the opposite shell margin, and height is measured perpendicular to the length line.
For more information see Shell Orientation and Measurements.