Fig. 296. Diagrammatic transverse sections, or outlines of the mouth, in certain Pteropod shells: A, B, Cleodora australis; C, C. pyramidalis; D, C. balantium; E, C. cuspidata. (After Boas.)
Fig. 297. Shells of thecosome Pteropods (after Boas). (1) Cleodora cuspidata; (2) Hyalaea trispinosa; (3) H. globulosa; (4) H. uncinata; (5) H. inflexa.
In certain cases (e.g. Cleodora, Hyalaea) the tube or cone is curiously modified. In the first place, its cross-section, originally {572} circular or nearly so, becomes flattened or compressed dorso-ventrally; and the angle, or rather edge, where dorsal and ventral walls meet, becomes more and more drawn out into a ridge or keel. Along the free margin, both of the dorsal and the ventral portion of the shell, growth proceeds with a regularly varying velocity, so that these margins, or lips, of the shell become regularly curved or markedly sinuous. At the same time, growth in a transverse direction proceeds with an acceleration which manifests itself in a curvature of the sides, replacing the straight borders of the original cone. In other words, the cross-section of the cone, or what we have been calling the generating curve, increases its dimensions more rapidly than its distance from the pole.
Fig. 298. Cleodora cuspidata.
In the above figures, for instance in that of Cleodora cuspidata, the markings of the shell which represent the successive edges of the lip at former stages of growth, furnish us at once with a “graph” of the varying velocities of growth as measured, radially, from the apex. We can reveal more clearly the nature of these variations in the following way which is simply tantamount to converting our radial into rectangular coordinates. Neglecting curvature (if any) of the sides and treating the shell (for simplicity’s sake) as a right cone, we lay off equal angles from the apex O, along the radii Oa, Ob, etc. If we then plot, as vertical equidistant ordinates, the magnitudes Oa, Ob ... OY, and again on to Oa′, we obtain a diagram such as the following (Fig. [299]); by {573} help of which we not only see more clearly the way in which the growth-rate varies from point to point, but we also recognise much better than before, the similar nature of the law which governs this variation in the different species.
Fig. 299. Curves obtained by transforming radial ordinates, as in Fig. [298], into vertical equidistant ordinates. 1, Hyalaea trispinosa; 2, Cleodora cuspidata.