Page:Scientific results HMS Challenger vol 18 part 2.djvu/334

1210 The genus Spongomelissa differs from its ancestral genus Lithomelissa only in the development of spongy framework in the shell-wall—a very rare production in the (compare Peripyramis, p. 1162, and Spongocyrtis, p. 1188).

1. Spongomelissa spongiosa, Haeckel.

Lithomelissa spongiosa, Bütschli, 1882, Zeitschr. f. wiss. Zool., vol. xxxvi. pp. 519, 539, Taf. xxxiii. fig. 25, a, b, c.

Shell of dense spongy structure, with a deep collar stricture. The subspherical cephalis and the truncate abdomen of nearly equal size, both with irregular delicate spongy framework. Mouth wide open. Cephalis with a large vertical apical and a small oblique frontal horn. Thorax with three very stout, three-sided prismatic widely divergent lateral wings, which are covered with numerous irregularly ramified branches; the spongy framework arises by communication of the delicate branches.

Dimensions.—Cephalis 0.04 long, 0.05 broad; thorax 0.04 long, 0.06 broad.

Habitat.—Fossil in Barbados.

Definition.— (vel Dicyrtida triradiata aperta) with three prominent lateral ribs on the thorax, alternating with three large holes (or thoracic gates). Cephalis with a horn.

The genus Clathrocanium is nearly allied to Dictyophimus and Lithomelissa, and, together with these two genera, may be regarded as surviving representatives of the oldest and most primitive forms of Dicyrtida. It differs from the latter mainly in the incomplete fenestration of the shell, three large interradial holes remaining between the three perradial thoracic ribs. It may therefore be derived either from Euscenium or from Tripospyris by the development of a terminal lattice-band between the three feet. Clathrocanium may be divided into two different subgenera: Clathrocanidium, with simple horn and smooth mouth, and Clathrocorona, with fenestrated horn and coronated mouth.

Definition.—Horn of the cephalis simple, not fenestrated. Peristome smooth.