13. Porodiscus helicoides, Haeckel.
All rings of the disk spirally convoluted around the central chamber; spiral line regular, simple. All rings nearly of the same breadth, connected by numerous alternating radial beams. Chambers little different in size, little longer than broad. Pores of equal size, regular, two on the breadth of each ring.
Dimensions.—Diameter of the disk (with ten rings) 0.2; breadth of each ring 0.01; pores 0.0025.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific; also fossil in the Tertiary rocks of the Mediterranean, Sicily, Oran, &c.
14. Porodiscus spiralis, Haeckel.
All rings of the disk spirally convoluted around the central chamber; spiral line regular, simple. All rings nearly of the same breadth, connected by numerous (twelve to sixteen) piercing radial beams. Chambers increasing in size from the centre towards the periphery. Pores irregular, of different size, one and a 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 0.004.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific; also fossil in the Tertiary rocks of Barbados, Sicily, Greece, &c.
15. Porodiscus operculina, Haeckel.
All rings of the disk spirally convoluted around the central chamber, spiral line regular, simple. Breadth of the rings and length of the chambers increasing in size from the centre towards the periphery. Most part of chambers twice as long as broad. Pores irregular, of very different sizes, two to three on the breadth of each ring.
Dimensions.—Diameter of the disk (with six rings) 0.16; breadth of the rings 0.012 to 0.016; pores 0.002 to 0.006.
Habitat.—Mediterranean (Messina), surface.
16. Porodiscus bilix, Haeckel.
All rings of the disk spirally convoluted around the large elliptical central chamber. Spiral convolutions of the inner rings simple, of the outer rings double; the latter twice as broad as the
