THE SHORE-SAND AT BOGNOR, SUSSEX. 221 



Discorbina orbicularis, Terquem, sp. 



Rosalina orbicularis, Terquein, 1876, Anim. sivr la plage de 



Dunkerque, fasc. 2, p. 75, pi. ix., fig. 4. 



Discorbina orbicularis, Brady, 1884, Report " Challenger" p. 647, 



pi. lxxxviii., figs. 4 — 8. 



,, „ Brady, 1887, Synopsis British Recent 



Foraminifera. 

 Rare. 



Discorbina parisiensis, d'Orbigny, sp. 

 Rosalina parisiensis, d'Orbigny, 1826, Ann. Sci. Nat., vol. vii., 



p. 271, No. 1 ; Modele, No. 38. 

 Discorbina parisiensis, Brady, 1884, Report " Challenger," p. 648, 



pi. xc, figs. 5, 6, 9 — 12. 

 ,, „ Brady, 1887, Synopsis British Recent 



Foraminifera. 

 Very common. The specimens are small and somewhat 

 variable, showing a great tendency to merge in the next species, 

 Discorbina wrightii, Brady, from which it is not always easily 

 distinguishable. I am of opinion that the two forms are very 

 closely allied, if, indeed, they are not identical. (Plate 12, 

 Figs. 4—7, and Plate 14, Fig. 5.) 



A very noticeable feature in connection with the occurrence 

 of these two forms at Bognor is the exceptionally large number 

 of " twin " or plastogamic * specimens which are to be found. 



* Plastogamy. — Plastogamy is the term used to describe the more or less 

 permanent union of two or more specimens of a forarninifer, and it 

 appears to be due "to the conjugation of two or more individuals, whose 

 nuclei are in a state of rest — i.e. not undergoing sub-division " (Chapman). 



The Forarninifei'a in general are reproduced by a process of sub-division. 

 The nucleus breaks up into a number of nucleoli. Each nucleolus takes to 

 itself a portion of the parent's protoplasm, secretes a test or shell, and 

 commences a separate existence, either within the parent shell or in its 

 immediate vicinity. This method of reproduction by means of the sub- 

 division of an active nucleus has been called " karyogamy." It is asexual, 

 and, continued indefinitely, would no doubt end in the weakening and 

 perhaps extinction of the stock. 



Plastogamy, on the other hand, probably partakes more or less of the 

 nature of a sexual conjugation, although there is at present no evidence 

 in support of the sexual theory, and its object may be the rejuvenescence 

 of a stock exhausted by long-continued karyogamy. It has been observed 

 in many genera ; I have myself notes of its occurrence in Penrroplis. 

 Textularia, Verneuilina, Bulimina, Lagma, MarginuUna, Cristellaria r 

 Patellinu, Bi.scorbbia, Rotalia, Nonionina, and Pohjstomella . It is, 



