Artiscida, Haeckel, 1881, Prodromus, p. 462.
Definition.—Prunoidea with an ellipsoidal twin-shell divided by an equatorial stricture into two communicating hemiellipsoidal or hemispherical chambers, without enclosed medullary shell. Central capsule ellipsoidal, with or without equatorial stricture.
The family Artiscida has a simple fenestrated outer shell, like that of the Ellipsida, but differs from these in the presence of an equatorial constriction, by which it assumes a characteristic twin form, somewhat similar to a violin (Pl. [39], figs. 9, 10). From the Cyphinida, which have the same form of the cortical shell, the Artiscida differ in the absence of the medullary shell. It is possible that the Artiscida are descended from the Cyphinida (by loss of the medullary shell), but it is more probable that they arise from the Ellipsida by an annular constriction in the equatorial plane (perhaps the formation of the shell originally took place while the central capsule was undergoing division). Both halves of the twin shell are always of the same size and form. Its outer surface is either smooth or covered with radial spines (Artiscus, Pl. [39], fig. 9). Sometimes on the opposite poles of the main axis are developed solid spines (Stylartus, Pl. [48], fig. 5) or hollow fenestrated tubes (Cannartus, Pl. [39], fig. 10).
The Central Capsule is either simply ellipsoidal, or has also an equatorial constriction, which divides it into two equal halves. It is constantly smaller than the surrounding shell, and separated from its inner surface by a thicker or thinner jelly-mantle, the calymma.
Synopsis of the Genera of Artiscida.
| On the poles of the main axis neither solid spines nor hollow tubes, | 152. Artiscus. |
| On the poles of the main axis two solid spines (or bunches of spines), | 153. Stylartus. |
| On the poles of the main axis two hollow fenestrated tubes, | 154. Cannartus. |
| On the poles of the main axis neither solid spines nor hollow tubes, | ||
| 152. Artiscus. | ||
| On the poles of the main axis two solid spines (or bunches of spines), | ||
| 153. Stylartus. | ||
| On the poles of the main axis two hollow fenestrated tubes, | ||
| 154. Cannartus. | ||
Genus 152. Artiscus,[[191]] Haeckel, 1881, Prodromus, p. 462.
Definition.—Artiscida without peculiar polar appendages (without strong solid spines or hollow fenestrated tubes on the poles of the main axis).
The genus Artiscus represents the most simple and primitive form of the Artiscida, and may therefore be regarded as the ancestral form of this subfamily. It resembles an ordinary double loaf, composed of two equal, nearly ellipsoidal fenestrated shells separated by the stricture in the equatorial plane. Artiscus can be derived from Cenellipsis simply by this equatorial constriction. But it can also be derived from Cyphanta by loss of the medullary shell.