496 THE VOYAGE OF H.M.S. CHALLENGER. 



13. Porodiscus helicoides, Haeckel. 



Discospira helicoides, Haeckel, 1862, Monogr. d. Eadiol., p. 514, Taf. xxix. fig. 7. 

 Trematodiscus helicoides, Haeckel, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 841. 



All rings of the disk spirally convoluted around the central chamber ; spiral line regular, 

 simple. All rmgs 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 O'Ol; pores 0-0025. 



Habitat. — Cosmopolitan ; Mediterranean, Atlantic, Pacific ; also fossil in the Tertiary rocks of 

 the Mediterranean, Sicily, Oran, &c. 



14. Porodiscus spiralis, Haeckel. 



Fliistrella spiralis, Wax&ahevg, 1840, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 210; 



MikrogeoJ., 1854, Taf. xix. fig. 62. 

 Discospira spiralis, Haeckel, 1862, Monogr. d. Piadiol., p. 513. 



AH 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) 016; breadth of each ring O'Ol ; pores 

 0-004 



Habitat. — Cosmopolitan ; Mediterranean, Atlantic, Indian, Pacific ; also fossil in the Tertiary 

 rocks of^Barbados, Sicily, Greece, &c. 



1 5. Porodiscus operculina, Haeckel. 



Discospira operculina, Haeckel, 1862, Monogr. d. Radiol., p. 514, Taf. xxix. fig. 8. 



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. 



Diseospiira bili.r, Stohr, 1880, Palfeontogr., vol. xxvi. p. 113, Taf. vi. fig. 3. 



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 



