Dimensions.—Diameter of the ring 0.04 to 0.05, of the central chamber 0.014.
Habitat.—Central Pacific, Stations 265 to 274, depths 2350 to 2950 fathoms.
Subfamily 2. Trematodiscida, Haeckel, 1862, Monogr. d. Radiol., p. 491 (sensu emendato et restricto).
Definition.—Porodiscida without radial appendages of the disk (solid spines or chambered arms on the margin), and without peculiar oscula on the margin of the disk, which is composed of two to four or more concentric rings.
Genus 214. Porodiscus,[[253]] Haeckel, 1881, Prodromus, p. 459.
Definition.—Porodiscida with simple circular disk, composed of several rings (without radial appendages or peculiar oscula on the margin of the disk).
The genus Porodiscus is, next to its ancestral form, Archidiscus, the simplest and most primitive form of the Porodiscida, from which all other genera of this family can be derived. The disk is quite simple, without any marginal appendages, composed of a variable number of rings, commonly of circular form, sometimes more or less polygonal, elliptical, or irregular. In my Monograph (1862, pp. 491, 513) I had separated the species, here united in Porodiscus, into two different genera: Trematodiscus with concentric rings, and Discospira with spiral rings. But the extended study of these very common forms in a great number of specimens in the Challenger collection has convinced me that the separation of those two genera cannot be maintained. In one and the same locality, where one single characteristic disk-form is very common, we find intermingled quite regular disks with only concentric, circular rings (Trematodiscus), and other disks with one single perfect spiral ring (Discospira); and between these a smaller number of specimens, in which the rings of the disk are partly concentric, partly spiral; either the rings of the central part of the disk are concentric, the outer spiral (Perispira), or the proportion is inverse (Centrospira); and sometimes the whole disposition of the concentric and spiral rings is irregular, and the rings often interrupted (Atactodiscus). Therefore it appears more natural to give to all these different forms only the value of subgenera of Porodiscus, as I have already proposed in my Prodromus (1881, p. 459). Even the numerous species of Porodiscus (mainly characterised by the equal or different breadth of the rings, and by the number, form, and size of the connecting radial beams and of the superficial pores) are for the most part very variable and hard to distinguish, as all those characters are not constant. Porodiscus is a quite "transformistic genus."
Subgenus 1. Trematodiscus, Haeckel, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 841.
Definition.—All rings of the disk concentric (commonly circular, rarely a little elliptical or polygonal).
1. Porodiscus orbiculatus, Haeckel.