Page:Scientific results HMS Challenger vol 18 part 1.djvu/791

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Definition.— with numerous solid radial spines on the margin of the disk (five to ten or more), all situated in the equatorial plane.

The genus Stylotrochus comprises those Spongodiscida in which the margin of the circular disk bears numerous radial spines. All these spines lie in the same equatorial plane, whilst in the following genus they are disposed over the whole surface of the disk. Stylotrochus corresponds to Astrocyclia among the Coccodiscida, to Stylodictya among the Porodiscida. The spongy framework of the disk is either quite irregular (Stylotrochiscus), or includes in the middle part some concentric circular rings (Stylospongia).

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

1. Stylotrochus arachnius, Haeckel.

Spongotrochus arachnius, Haeckel, 1862, Monogr. d. Radiol., p. 464.

Spongy framework of the whole disk irregular. Eight to twelve marginal spines very long and thin, bristle-shaped, twice to four times as long as the diameter of the disk. (Very similar to the common Stylodictya arachnia, but without concentric circular rings and sieve-plates, with quite irregular network of fine bars.)

Dimensions.—Diameter of the disk 0.12 to 0.15; length of the radial spines 0.2 to 0.6, basal breadth 0.001.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, surface; also fossil in Tertiary rocks of Barbados and the Mediterranean.

2. Stylotrochus craticulatus, Haeckel.

Spongotrochus craticulatus, Stöhr, 1880, Palæontogr., vol. xxvi. p. 118, Taf. vi. fig. 12.

? Spongodiscus aculeatus, Ehrenberg, 1854, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 246.

Spongy framework of the whole disk irregular. Sixteen to twenty short marginal spines (twice to four times as long as the diameter of one mesh of the framework), free prolongations of internal radial beams which arise from the darker centre of the disk. (The interruption of the disk-margin on one point of its circumference, figured by Stöhr as osculum or "Mündungs-Oeffnung," is probably an accidental abnormality; I did not find it in other specimens.)