{lxxv}A. The group Adelacantha consists only of the suborder #Actinelida#, with the three families Astrolophida, Litholophida, and Chiastolida (p. 728, Pl. 129, figs. 1-3); the number of the radial spines is very different and variable, sometimes only from ten to sixteen, but usually from thirty to fifty, and often more than one hundred; they are generally irregularly distributed, and not as in the second main division. This latter, the Icosacantha, always possesses _twenty_ radial spines, which are regularly disposed according to a constant law, the so-called "Müllerian" or "Icosacanthan" law; the twenty spines are always so placed between the poles of a spineless axis that they form five zones each of four spines; the four spines of each zone are equidistant from each other, and also from the same pole, and alternate with those of the neighbouring zones, so that the whole twenty lie in four meridian planes, which cut out an angle of 45° (compare pp. 717-722, Pls. 130-140). In spite of the manifold variations in form which are developed in the Icosacantha, they may all be derived from a common stem-form, _Acanthometron_ (p. 742), since the law of distribution of the twenty spines is constantly inherited. B. An exception is found in the peculiar family Sphærocapsida (p. 797, Pl. 133, figs. 7-11; Pl. 135, figs. 6-10). Here the shell is composed of innumerable small, perforated plates, which arise on the surface of the calymma independently of the spines.