Page:Scientific results HMS Challenger vol 18 part 1.djvu/101

Rh development of the vertical main axis, the ellipsoidal Belonaspida have arisen from the spherical Dorataspida among the (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 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.

121. The Discoid Skeletons or Lattice-Discs.—The "lattice-discs" or Discoid skeletons are characteristic of the group, and have arisen from the lattice-spheres of the  by a less development of one axis, which is the main axis of the body, and is probably usually vertical; its two poles are always equal. The Discoid lattice-shell is either a biconvex lens (with a thin margin), or a plane disc (a shortened cylinder with thick margin), or some form intermediate between the two. All Discoid shells show a horizontal median plane or equatorial plane, by which they are divided into two equal halves, an upper and lower; the margin of the lens itself is originally the equator. The main axis, the shortest of all the axes of the shell, stands vertically in the centre of the equatorial plane. Among the Discoid shells rarely occur (Aulophacus), as also among the  (Hexalaspida).

122. The Larcoid Skeleton or Lentelliptical Lattice-Shell.—The lentelliptical lattice-shells, which may be shortly designated "Larcoid," are especially characteristic of the, a large order of (pp. 599-715; Pls. 9, 10, 49, 50). In addition they recur among the, in the small family Hexalaspida (p. 872, Pl. 139), and the family Diploconida (p. 881, Pl. 140), which is derived from it. These lentelliptical lattice-shells are all characterised by the clear differentiation of three unequal, but isopolar dimensive axes, i.e., the three geometrical axes, perpendicular to one another, which determine the form of the shell, are of unequal length; the two poles of each are, however, equal. The geometrical ground-form is, therefore, a triaxial ellipsoid (§ 34). In the rich order the lentelliptical lattice-shell shows many variations in its development.

123. The Cyrtoid Skeleton.—Cyrtoid skeletons are those lattice-shells which possess a vertical main axis with two different poles (Monaxonia allopola); the upper pole is usually termed the apical, the lower the basal. Such Cyrtoid shells are characteristic of the great majority of the or  (and especially of the ); they are also found in a large division of the  (the ), and in some. In general the manifold Cyrtoid shells may be divided into two large groups, those with one and those with several chambers. The monothalamous Cyrtoid shells are usually ovoid, conical, cap- or helmet-shaped; their