123 



species, as well as in 0. adunca, all the septa are pierced with them 

 in considerable numbers. In the latter instance especially, the whole 

 structure becomes little more than a convoluted shell enclosing a 

 net-work of flattened, calcareous pillars, arranged at right angles to 

 each other. Such close typical resemblances of structure and deve- 

 lopment as are here shown to exist, satisfy me as to the correctness 

 of M. D'Orbigny's opinion, that Orbiculina complanata and its allies 

 are true Foraminifera, and not Zoophytes. 



In my memoir on Polystomella crispa* I pointed out the existence 

 in that species of a hard, central mass of translucent carbonate of 

 lime, occupying each umbilical region, the surface of which was pit- 

 ted by small depressions. I referred to these depressions as being 

 probably the external orifices of canals communicating with the 

 deeper and more central convolutions of the organism. I had not, 

 however, been able to demonstrate the actual existence of such pas- 

 sages. In the same memoir I also referred to the difficulty of 

 accounting for the external additions made to the surface of each 

 calcareous mass, and suggested " that the central nucleus of lime 

 may have been thickened through the instrumentality of the pseudo- 

 podia, which appear to penetrate it from the inner convolutions, and 

 which, if this explanation prove to be correct, will thus possess an 

 additional function to those already noticed. This, however, is a 

 mere suggestion, arising from the difficulty of accounting for the 

 appearance. The only other explanation which appears plausible, 

 is, that the newly-formed segment, before enclosing itself in its cal- 

 careous cell, extends itself over the central nucleus, and there, gra- 

 dually receding to what constitutes its permanent limit, leaves a 

 calcareous layer behind it." 



Vertical sections of P. crispa which I now possess, lead me to 

 conclude that the latter of these explanations approaches nearest 

 to the truth. We have seen that the structure of Amphistegina gib- 

 bosa could be accounted for in no other way, inasmuch as in that 

 species no pseudopodia traverse the greater portion of each central 

 nucleus, and consequently they could scarcely be the instruments em- 

 ployed in making the external additions. Fig. 19 represents the um- 

 bilical region from the upper half of a vertical section of Polystomella 

 crispa. 19, a, is one of the internal cells near the centre, if not the 

 primordial one, which I believe it to be. 19, b, are portions of cells 

 belonging to successive convolutions. 19, c, are the small apertures 



* • Transactions of the Microscopical Society of London,' vol. ii. p. 170. 



