Page:Scientific results HMS Challenger vol 18 part 1.djvu/706

498 20. Porodiscus deformis, Haeckel.

Discospira deformis, Stöhr, 1880, Palæontogr., vol. xxvi. p. 114, Taf. vi. fig. 6.

Rings of the disk irregular, partly concentric, partly spiral, often interrupted, increasing in breadth from the centre. Radial beams not piercing; pores regular, circular, two to six on the breadth of the different rings. (Very variable and irregular, sometimes more spiral, at other times more concentric, but always with equal pores.)

Dimensions.—Diameter of the disk (with four rings) 0.18; breadth of the first ring 0.014, of the second 0.03, of the fourth 0.06; pores 0.005.

Habitat.—Fossil in the Tertiary rocks of Barbados and Sicily; living in the Central Pacific, Stations 266 to 268, depths 2700 to 2900 fathoms.

21. Porodiscus irregularis, n. sp. (Pl. 41, fig. 7).

Atactodiscus irregularis, Haeckel, 1881, Prodromus, p. 459.

Perispongidium irregulare, Haeckel, 1878, MS. et Atlas (pl. xli. fig. 7).

Rings of the disk irregular, partly concentric, partly spiral, often interrupted, with nearly equal breadth. Radial beams not piercing; pores irregular, roundish, two to four on the breadth of each ring; network in the periphery of the disk spongy. (Very variable and irregular, sometimes more spiral, at other times more concentric; disk in the peripheral part often more or less spongy. Differs from the preceding by the equal breadth of the rings and the different size of the pores.)

Dimensions.—Diameter of the disk (with six rings) 0.15; breadth of each ring 0.011; pores 0.002 to 0.004.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface; also fossil in Barbados.

Definition.— with a simple circular disk (without radial spines and chambered arms), surrounded on the margin by a thin porous (but not chambered) equatorial girdle.

The genus Perichlamydium differs from Porodiscus only in the development of a thin, porous, equatorial girdle, which surrounds the circular margin of the chambered disk. This girdle lies in the equatorial plane of the lenticular disk, and represents a very delicate siliceous plate, perforated by numerous small pores. Sometimes the proximal part of the girdle is ribbed by thin radial beams, the distal prolongations of the radial rods of the central disk. If these ribs reach the margin of the girdle and are prominent over it, Perichlamydium passes over into Stylochlamydium.