In Fig. 6, F represents the tangential force that tends to cause skidding.
From this simple lattice sphere all other Sphroidea can be derived either by radial or by tangential growth.
The tangential needles always seem to have the same shape as I have accurately described, in 1862, of Aulacantha scolymantha.
They are, in short, mountains of elevation, ridged up by tangential thrusts.