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.
All rings of the disk circular, concentric, of equal breadth, connected by numerous alternating radial beams. Chambers differing little in size, about as large as the central chamber. Pores regular, circular, two to two and a half on the breadth of each ring.
Dimensions.—Diameter of the disk (with nine rings) 0.18; breadth of each ring 0.01; pores 0.003.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, surface; also fossil in Tertiary rocks of Barbados, Sicily, and Nicobar.
2. Porodiscus concentricus, Haeckel.
Trematodiscus concentricus, Stöhr, 1880, Palæontogr., vol. xxvi. p. 108.
Flustrella concentrica, Ehrenberg, 1838, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 132; Ibid., 1875, p. 72, Taf. xxii. fig. 13.
? Flustrella concentrica, Ehrenberg, 1854, Mikrogeol., Taf. xix. fig. 61, Taf. xx. fig. 42, Taf. xxi. fig. 51, Taf. xxxvi. fig. 29.All rings of the disk circular, concentric, of equal breadth, connected by numerous piercing radial beams. Chambers different in size, increasing from the centre towards the periphery. Pores regular, circular, one and half to two on the breadth of each ring.
Dimensions.—Diameter of the disk (with eight rings) 0.16; breadth of each ring 0.01; pores 0.003.
Habitat.—Fossil in many Tertiary rocks—Barbados, Sicily, Greece, &c.
