492 THE VOYAGE OF H.M.S. CHALLENGEE. 



these a smaller number of specimens, in which the rings of the disk are partly con- 

 centric, 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 (Atactodisciis). 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. — Ail rings of the disk concentric (commonly circular, rarely a little 

 elliptical or polygonal). 



1. Porodiscus orbicidatus, Haeckel. 



Trematodiscus orbicidatus, Haeckel, 1862, Monogr. d. EadioL, p. 492, Taf. xxix. fig. 1. 

 Trematodiscus orhiculatus, Stbhr, 1880, Palgeontogr., vol. xxvi. p. 108. 



All rings of the disk circular, concentric, of equal breadth, connected by numerous alternating 

 radial beams. Chambers differing Uttle in size, about as large as the central chamber. Pores 

 regular, chcular, two to two and a half on the breadth of each ring. 



Dimensions. — Diameter of the disk (with nine rings) 018 ; breadth of each ring O'Ol ; 

 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, Haeckel, 1862, Monogr. d. EadioL, p. 493. 

 Trematodiscus concentricus, Stohr, 1880, Palseontogr., vol. xxvi. p. 108. 



Flustrella concentrica, Ehrenberg, 1838, Abhandl. d. k. Akad. d. Wiss. LerHn, p. 132; Ibid., 

 1875, p. 72, Taf. xxii. fig. 13. 

 1 Flustrella co7icentrica, 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, increasiag from the centre towards the periphery. Pores 

 regular, circular, one and half to two on the breadth of each ring. 



Dimensions. — Diameter of the cUsk (with eight rings) 016; breadth of each ring O'Ol; pores 0-003. 



Habitat. — Fossil in many Tertiary rocks — Barbados, Sicily, Greece, &c. 



