Species in which the radii are all introduced at an early
stage of growth (many species of Cardium, Pecten, Lima)
or in which the radii multiply by regular dichotomy would


ORIGIN AND SIGNIFICANCE OF SPINES 15


show, of course, only the longitudinal series, for at the
margin of the shell the radii would be of the same size and
age, and the spines uniform.
The foregoing example illustrates an important principle


SUS LUSUESILaS


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AZA SP PAY 29ST TASAS 34


Ficure 19.— Sector; showing in diagram the multiplication of radiating
lines by interpolation. The two primary radii (1, 1) are the only ones continuing
through the whole four zones. The first zone has 2 radii ; the second, 5; the third,
11; and the fourth, 23.
