120. _The Prunoid Skeleton or Lattice-Ellipsoid._--The "lattice-ellipsoids" or Prunoid skeletons have arisen from the lattice-spheres or Sphæroid skeletons by more energetic growth and elongation of one axis; this is the main axis of the body and is probably always vertical; its two poles are commonly equal. The Prunoid skeleton is either a true ellipsoid in the geometrical sense or an "endellipsoidal polyhedron" (_i.e._, a polyhedron, all the angles of which lie in an ellipsoidal surface). By further elongation of the main axis, the ellipsoidal form passes over into the cylindrical, the polar surfaces of the cylinder being usually rounded, rarely truncated. The rich order #Prunoidea# (pp. 284-402) contains numerous modifications of this form of shell which arise on the one hand by the formation of transverse constrictions, on the other by the apposition of concentric secondary shells. In respect of the latter, simple and compound Prunoid shells can be distinguished as in the case of the Sphæroid shells. In the compound Prunoid shells either all the concentric lattice-shells may be ellipsoidal or the inner may be spherical. More important differences are found in the transverse annular constrictions, which give the Prunoid skeleton a segmented appearance; in this respect, three principal forms may be distinguished (p. 288):--(A) _Monoprunida_, with unsegmented shell, having no transverse constriction (Pls. 15-17); (B) _Dyoprunida_, having a shell with two segments and one (equatorial) transverse constriction (Pl. 39); (C) _Polyprunida_, with three or more parallel transverse constrictions, by means of which the shell is divided into four or more segments (Pl. 40). In the same manner as the #Prunoidea# have arisen from the #Sphæroidea# among the SPUMELLARIA by greater {lxxxi}development of the vertical main axis, the ellipsoidal Belonaspida have arisen from the spherical Dorataspida among the ACANTHARIA (p. 859; Pl. 136, figs. 6-9; Pl. 139, figs. 8, 9). The main axis of the ellipsoid in this case is always occupied by the opposite equatorial spines of the hydrotomical axis (pp. 719, 860). In the legion PHÆODARIA a similar prolongation of the main axis rarely occurs; it is found, however, in _Aulatractus_ (Pl. 111, figs. 6, 7), the lattice-shell of this Aulosphærid being sometimes truly fusiform, sometimes rather ellipsoidal or even double-conical.