152 J. J. LISTER. 



the history of the nucleus, as it is so large that each specimen 

 must be examined by means of sections ; moreover, the para- 

 sitic organisms that it harbours, and the abundance of large 

 food particles contained in the protoplasm, often make it a 

 difficulty to distinguish the nuclear elements proper to the 

 organism itself. 



As shown by Brady (2), the phenomenon of dimorphism is 

 also presented by this species. The individuals fall into two 

 groups, (1) those whose centre is occupied by a "primitive disc" 

 (fig. 45), which consists of the large "primoi-dial chamber" 

 (megalosphere), together with the second or "circumambient" 

 chamber, and (2) those in which the small chambers or chamber- 

 lets, as they are called by Carpenter, are continued to the 

 centre. These evidently correspond with the megalospheric 

 and microspheric forms of Polystomella and other Foraminifera. 



It will be convenient to deal with the Tongan and Fijian 

 specimens fii-st, and with those from Celebes afterwards. 



In the Fijian and Tongan specimens that I have examined 

 there is a marked difference in size between the two forms, the 

 microspheric attaining a much larger size than the megalo- 

 spheric. Thus, among my thirty- three largest specimens of the 

 megalospheric form from Tonga, the number of rings of small 

 chambers is under thirty in all cases except one, in which it is 

 thirty-eight ; among thirteen specimens from Fiji, the greatest 

 number is thirty-three. In the specimens of the microspheric 

 form, on the other hand, the number of rings of chambers varies 

 from 79 to 110, as they are seen in sections of five specimens in 

 the Brady Collection, and I have many examples from Tonga 

 which are equally large. 



The shells of this species, as is well known, are bi-concave, 

 being thin at the centre and thick at the margin. As Brady 

 points out, the central part of the microspheric form, composed, 

 as above stated, of small chambers, is not more than one-third 

 the thickness of the primitive disc which occupies the centre of 

 the megalospheric form. 



As the microspheric form increases in size, it frequently 

 happens that the edge of the disc becomes double, so that a 



