Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface; also fossil in Barbados.
Genus 215. Perichlamydium,[[254]] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.
Definition.—Porodiscida 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.
1. Perichlamydium praetextum, Ehrenberg.
Perichlamydium praetextum, Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 43; Mikrogeol., 1854, Taf. xxii. fig. 21 (non 20).
Perichlamydium praetextum, Haeckel, 1862, Monogr. d. Radiol., p. 495.
All rings of the disk (three to four) concentric, circular, of equal breadth, with interrupted (not piercing) radial beams. Equatorial girdle without radial beams, nearly as broad as the disk; its circular pores of the same size as those of the disk; about two pores on the breadth of each ring.
Dimensions.—Diameter of the disk (without the girdle, with four rings) 0.11; breadth of each ring 0.012; breadth of the girdle 0.06 to 0.1; pores 0.004.
Habitat.—Cosmopolitan; Atlantic, Indian, Pacific, surface; also fossil in the Tertiary rocks of Barbados and Sicily.