Quarterly Journal of Microscopical Science, Vol s3-94, 57-70, Copyright © 1953 by Company of Biologists

The Shell in the Lamellibranchia

¹ Department of Zoology, University of Glasgow

1. In the Lamellibranchia, the direction of growth at any region of the valve margins may be resolved into: (a) a radial component radiating from the umbo and acting in the plane of the generating curve; (b) a transverse component acting at right angles to the plane of the generating curve; (c) a tangential component acting tangentially to, and in the plane of, the generating curve. The radial component is always present and affects the form of both valves while the transverse component may be reduced or absent in one valve.

2. The lamellibranch mantle/shell is orientated with reference to the normal axis. This normal axis follows that sector of the shell secreted by the normal zone of the mantle edge (i.e. where the effect of the transverse component is greatest) and passes through the umbo, the normal zone and the point at which the greatest transverse diameter of the shell intersects the surface of the valves.

3. The form of the shell valves should be considered with reference to: (a) the outline of the generating curve; (b) the spiral angle of the normal axis; (c) the form (i.e. planospiral or turbinate spiral) of the normal axis.

4. The ‘deflection’ anteriorly of the umbones and the splitting of the anterior portion of the ligament in many bivalves is a consequence of a tangential component affecting the form of the valves.

5. In both gastropods and lamellibranchs, the turbinate spiral shell is the resultant of two differential growth ratios. In the Gastropoda, however, both ratios act perpendicularly to the plane of the generating curve, while in the Lamellibranchia, one acts perpendicularly to this plane while the other acts in the plane of the generating curve.