Page:Scientific results HMS Challenger vol 18 part 1.djvu/784

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Definition.— with a simple circular disk, without radial appendages and without an equatorial girdle on the margin.

The genus Spongodiscus represents the most simple and primitive form of the Spongodiscida, or of those in which the central disk is more or less spongy, composed of an irregular fine framework. In my Monograph (1862, pp. 452, 460, 469) I had separated the true Spongodiscus (first described by Ehrenberg, loc. cit.) and the Spongocyclia; the former being characterised by the irregular spongy framework of the whole disk, whilst in the latter this framework includes in the central part some concentric circular rings (approaching Porodiscus). In the same way afterwards Stöhr (1880, loc. cit.) separated the genus Spongospira as spongy disks, which include in the central part some spiral convolutions. But as these differences are rather inconstant and not sharply discernible, I think it now better to regard these three forms as subgenera of Spongodiscus. All three have the common simple circular disk, without any marginal appendages.

Definition.—Spongy framework of the disk quite irregular, without concentric rings or spiral convolutions.

1. Spongodiscus mediterraneus, Haeckel.

Spongodiscus mediterraneus, Haeckel, 1862, Monogr. d. Radiol., p. 461, Taf. xii. figs. 14, 15.

Spongy disk plain on both sides, with quite an irregular framework, without concentric rings and without radial piercing beams. Texture everywhere uniform; meshes eight to ten times as broad as the bars.

Dimensions.—Diameter of the disk 0.08 to 0.24, of the meshes 0.005 to 0.008.

Habitat.—Mediterranean (Messina), Haeckel; also fossil in the Tertiary rocks of Sicily (Grotte), Stöhr.

2. Spongodiscus radiatus, n. sp.

Spongy disk plain on both sides, with quite irregular framework, without concentric rings, but with numerous piercing radial beams which are not prolonged into marginal spines. Texture everywhere uniform; meshes once and a half to twice as broad as the bars.

Dimensions.—Diameter of the disk 0.12 to 0.15, of the meshes 0.002 to 0.003.

Habitat.—Pacific, central area, Station 268, depth 2900 fathoms.