498 ,. THE VOYAGE OF H.M.S. CHALLENGER. 



20. Porodiscus deformis, Haeckel. 



Discospira deformis, Stohr, 1880, Palseontogr., vol. xxvi. p. 114, Taf. vi. fig. 6. 



Eings of the disk irregular, partly concentric, partly spiral, often interrupted, increasing in 

 breadth from the centre. Eadial beams not piercing; pores regular, circular, two to six on the 

 breadth of the different rings. (Very variable and irregular, sometimes more spiral, at other times 

 more concentric, but always with equal pores.) 



Dimensions. Diameter of the disk (with four rings) O18 ; breadth of the first ring 0'014, of 

 the second 0'03, of the fourth O06 ; pores 0'005. 



Habitat. Fossil in the Tertiary rocks of Barbados and Sicily ; living in the Central Pacific, 

 Stations 266 to 268, depths 2700 to 2900 fathoms. 



21. Porodiscus irregularis, n. sp. (PI. 41, fig. 7). 



Atactodiscus irregularis, Haeckel, 1881, Proilromus, p. 459. 

 Perispongidium irregulare, Haeckel, 1878, MS. et Atlas (pi. xli. fig. 7). 



Eings of the disk irregular, partly concentric, partly spiral, often interrupted, with nearly equal 

 breadth. Eadial beams not piercing ; pores irregular, roundish, two to four on the breadth of each 

 ring ; network in the periphery of the disk spongy. (Very variable and irregular, sometimes more 

 spiral, at other times more concentric ; disk in the peripheral part often more or less spongy. 

 Differs from the preceding by the equal breadth of the rings and the different size of the pores.) 



Dimensions. Diameter of the disk (with six rings) 015 ; breadth of each ring O'Oll ; pores 

 0-002 to 0-004. 



Habitat. Cosmopolitan ; Mediterranean, Atlantic, Pacific, surface ; also fossil in Barbados. 



Genus 215. Perichlamydium, 1 Ekrenberg, 1847, Monatsber. d. k. preuss. 



Akad. d. Wiss. Berlin, p. 54. 



Definition. P orodiscida with a simple circular disk (without radial spines and 

 chambered arms), surrounded on the margin by a thin porous (but not chambered) 

 equatorial girdle. 



The genus Perichlamydium differs from Porodiscus only in the development of a 

 thin, porous, equatorial girdle, which surrounds the circular margin of the chambered 

 disk. This girdle lies in the equatorial plane of the lenticular disk, and represents a 

 very delicate siliceous plate, perforated by numerous small pores. Sometimes the 

 proximal part of the girdle is ribbed by thin radial beams, the distal prolongations of 

 the radial rods of the central disk. If these ribs reach the margin of the girdle and 

 are prominent over it, Perichlamydium passes over into Stylochlamydium. 



1 Perichlamydium Shell surrounded by a mantle ; m^l, x^xftttiiw. 



