Shell biconvex, lenticular, with rounded circular margin; its horizontal diameter three times as great as its thickness (or its vertical main axis). Radial tubes cylindro-conical, straight, with six to nine triradiate verticils, those of the margin of the lens twice as long as the smooth tangential tubes, the others shorter.
Dimensions.—Horizontal diameter of the lens 5.5 mm., vertical main axis 0.2; length of the radial tubes 0.2, breadth 0.01.
Habitat.—Indian Ocean, Belligemma, Ceylon (Haeckel), surface.
2. Aulophacus amphidiscus, n. sp. (Pl. [111], fig. 5b).
Shell discoidal, slightly biconvex, with rounded circular margin; its horizontal diameter five times as great as its thickness. Radial tubes cylindrical, covered with numerous short thorns, those of the margin of the lens three times as long as the smooth tangential tubes, the others shorter.
Dimensions.—Horizontal diameter of the lens 6.0, vertical main axis 1.2; length of the radial tubes 0.2, breadth 0.005.
Habitat.—Indian Ocean, Madagascar (Rabbe), surface.
Genus 687. Aulatractus,[[306]] n. gen.
Definition.—Aulosphærida with triangular meshes in the network, the tangential tubes of which form an ellipsoidal or spindle-shaped, simple lattice-shell, with prolonged vertical main axis. Radial tubes arise at its nodal points.
The genus Aulatractus differs from all the other Aulosphærida in the peculiar prolongation of the vertical axis of the lattice-sphere, by which the latter becomes ellipsoidal or spindle-shaped. The triangular meshes of the regular lattice-work become usually more or less irregular and polygonal at the two poles of the elongated main axis. Aulatractus seems to be developed from Aulosphæra in the opposite way from Aulophacus, the former by elongating, the latter by shortening of the vertical main-axis.