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



7. Porodiscus ellipticus, Haeckel. 



Trematodiscus ellipticus,' Siohr, 1880, Paieontogr., vol. xxvi. p. 108, Taf. iv. fig. 16. 



All rings of the disk concentric, either circular or a little elliptical, connected by eight piercing 

 radial beams ; central chamber elliptical, of the same breadth as the first ring, broader than the 

 following rings, the breadth of which decreases towards the periphery. Pores irregular, in the 

 inner rings twice as broad as in the outer (third) ring, four to five on the breadth of each ring. 



Dimensions. Diameter of the disk (with three rings) 018 ; breadth of the inner rings 0'03, of 

 the outer 0'02 ; pores 0'003 to O006. 



Habitat. Fossil in Tertiary rocks of Sicily, Grotte, Stohr. 



8. Porodiscus heterocydus, Haeckel. 



Trematodiscus heterocydiis, Haeckel, 1862, Monogr. d. EadioL, p. 493, Taf. xxix. fig. 3. 

 IFlustrella cyclia, Harting, 1863, Fauna Banda-Zee, p. 11, pi. i. fig. 19. 

 Trematodiscus heterocijdus, Stohr, 1880, Palaeontogr., vol. xxvi. p. 108. 



All rings of the disk concentric, circular, connected by numerous radial beams, which are 

 partly piercing, partly alternating. The breadth of the rings increases gradually from the centre 

 towards the periphery, corresponding also to the size of the pores ; two to three circular pores on the 

 breadth of each ring. 



Dimensions. Diameter of the disk (with six rings) 017 ; breadth of the second ring 0-007, of 

 the sixth 0-02 ; pores 0-002 to O'OOG. 



Habitat. Cosmopolitian ; Mediterranean, Atlantic, Pacific, surface; also fossil in Tertiary 

 rocks of Barbados and the Mediterranean. 



9. Porodiscus quadrigatus, n. sp. (PI. 41, fig. 3). 



All rings of the disk concentric, of nearly equal breadth, connected by four interradial beams, 

 perpendicular one to another. The first ring (surrounding the central chamber) with eight 

 chambers, the second only with four (alternating with the four radial beams). Size of the four 

 chambers of each ring increases much towards the periphery. Pores regular, circular, about two 

 on the breadth of each ring. 



Dimensions. Diameter of the disk (with six rings) 018; breadth of each ring 0'05 ; 

 pores 0-004. 



Habitat. Pacific, central area, Station 271, depth 2425 fathoms. 



Subgenus 2. Perispira, Haeckel, 1881, Prodromus, p. 459. 



Definition. The inner rings of the disk concentric (commonly circular) ; the outer 

 rings spirally convoluted. 



