It may be taken that the general tendency of the evolution of surface ornament (and therefore of the radioles) in the Holectypoida was a dual one. The largo tubercles become gradually restricted to the adoral surface, and often reduced in number, while the actual density of the tuberculation increases. This increase is attained, either by a simple multiplication of the normal vertical series, as in Discoïdea (this being sometimes associated with the transformation of granules to tubercles at the ambitus), or by a more uniform distribution of those series over the area of the plates, and the frequent substitution of two tubercles in the place of one, as in Conulus. Both these types of change lead away from the primitive, ‘Regular’ condition of tuberculation, and trend towards the Clypeastroid and Cassidulid arrangement.