The greatest diameter of a Pecten is nearly always the height. It is easier to visualize the size of a shell from this measurement than from the length. Consequently in tabulating the dimensions of the species the height is first given in millimetres, followed by the length as a percentage of the height. Since in the vast majority of the ordinary species the shell is, except for the auricles of the right valve, subequilateral, no obliquity measurement is necessary as with the Limidae. Another redundant measurement in dealing with Jurassic Pectinidae is the inflation. This dimension is always such a small percentage of the height that accuracy of measurement without special apparatus is a matter of difficulty unless both valves happen to be joined, closed, and free from matrix. Even such differences as do exist can rarely be detected in allied species, such as might be liable to confusion in other respects.