Page:Scientific results HMS Challenger vol 18 part 1.djvu/568

360 The Cortical Shell is commonly simple (Pl. 39, figs. 12, 18), sometimes composed of two concentric shells (Pl. 39, fig. 13), rarely of three. As in the Artiscida, also in the Cyphinida, from both poles of the main axis solid spines or hollow fenestrated tubes are often developed (Pl. 39, figs. 14, 16-18).

The Central Capsule of the Cyphinida (Pl. 39, fig. 13) is of the same form as in the Artiscida, generally ellipsoidal, but with an equatorial ring-like stricture, which divides it into two equal halves. It encloses the simple or double medullary shell, and is perforated by the radial beams starting from this. From the inner surface of the surrounding cortical shell it is separated by a thicker or thinner jelly-layer, the calymma. (Compare also Taf. xxii. fig. 14 of my Monograph, 1862.)

Definition.— with simple cortical shell and simple medullary shell, without polar spines or tubes.

The genus Cyphanta is the most simple of all Cyphinida, and can be regarded as the common ancestral form of this family. It may be derived phylogenetically from Druppula by a ring-like constriction in the equatorial plane of the ellipsoidal cortical shell, or from Artiscus by secondary formation of a central (spherical or ellipsoidal) medullary shell.

Definition.—Surface of the cortical shell smooth, without spines or thorns.