traces on the surface of the operculum {522} (Fig. 264, 1), which traces have the form of curved lines in Turbo, and of straight lines in (e.g.) Nerita (Fig. 264, 2); that is to say, apart from the side constituting the outer edge of the operculum (which side is always and of necessity curved) the successive increments constitute curvilinear triangles in the one case, and rectilinear triangles in the other. The sides of these triangles are tangents to the spiral line of the operculum, and may be supposed to generate it by their consecutive intersections.
In a number of such opercula, Moseley measured the breadths of the successive whorls along a radius vector14, just in the same way as he did with the entire shell in the foregoing cases; and here is one example of his results.
| Distance | Ratio | Distance | Ratio | Distance | Ratio | Distance | Ratio |
|---|---|---|---|---|---|---|---|
| ôñ24 | ôñ16 | ôñ2ã | ôñ18 | ||||
| 2ôñ28 | 2ôñ31 | 2ôñ30 | 2ôñ30 | ||||
| ôñ55 | ôñ37 | ôñ6ã | ôñ42 | ||||
| 2ôñ32 | 2ôñ30 | 2ôñ30 | 2ôñ24 | ||||
| 1ôñ28 | ôñ85 | 1ôñ38 | ôñ94 |
The ratio is apôÙproxôÙiôÙmateôÙly constant, and this spiral also is, therefore, a logarithmic spiral.