1. Mesocena circulus, Ehrenberg.
Rings of the skeleton circular, smooth or slightly denticulate on the outer margin, without spines. This simple form is probably the ancestral form of the Dictyochida.
Dimensions.—Diameter of the rings 0.02 to 0.03, thickness of the hollow rod 0.001 or less.
Habitat.—Fossil in Tertiary rocks of Barbados and of the Mediterranean (Ægina, Greece; Caltanisetta, Sicily).
2. Mesocena annulus, n. sp.
Rings of the skeleton elliptical, smooth or slightly denticulate on the outer margin, without spines. This species is possibly identical with Mesocena crenulata of Ehrenberg, of which, however, no figure is given. The major axis of the ellipse is one and a half times as long as the minor.
Dimensions.—Diameter of the rings in the major axis 0.03, in the minor 0.02; thickness of the bars 0.002.
Habitat.—Tropical Atlantic, Station 347, depth 2250 fathoms.
3. Mesocena diodon, Ehrenberg.
Rings of the skeleton elliptical, smooth, with two opposite spines on the poles of the major axis, which is about one and one-third as long as the minor axis.
Dimensions.—Diameter of the rings in the major axis 0.04, in the minor 0.03; length of the spines 0.01.
Habitat.—Fossil in Barbados, and in North America (Virginia).
4. Mesocena triangula, Ehrenberg.
Mesocena triangula, Ehrenberg, 1854, Mikrogeol., Taf. xxii. fig. 41.
Dictyocha triangula, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 46.
Lithocircus triangularis, Stöhr, 1880, Palæontogr., vol. xxvi. p. 121, Taf. vii. fig. 10.Rings triangular, with small peripheral thorns, and three larger teeth on the corners of the equilateral triangle.
Dimensions.—Diameter of the rings 0.03 to 0.05, length of the spines 0.01.
Habitat.—Fossil in Tertiary deposits of the Mediterranean; Caltanisetta, Sicily (Ehrenberg, Stöhr).
