126 



THE FORAMINIFERA 



development ; thus a one-nucleated individual and an individual 

 whose nucleus had begun to divide would not unite. 



In Discorbina a similar process was observed ; but in this case the 



two individuals came together 

 base to base, and the pair 

 wandered about for a consider- 

 able time before the young 

 were produced. In some cases 

 a deposit of lime between the 

 opposed bases occurred in the 

 interval, so that after the escape 

 of the young the empty parent 

 shells remained united together. The 

 remarkable pairs of shells which have 

 been observed in Discorbina, Textularia, 

 and Bulimina (Fig. 54) are, probably, 

 thus explained. 



Fio. 54. 



Paired tests of a species of Buli- 

 mina from Delos. In a the paired 

 individuals are of equal, in b of 

 very unequal size. From speci- 

 mens kindly given me by Mr. H. 

 Sidebottom. 



ORDER Nummulitidea. 



The members of this order are dis- 

 tinguished by their bilaterally sym- 

 metrical tests, which in the early stages 

 or throughout growth are arranged 

 on the spiral plan ; by the double character of the septa between 

 the chambers, containing branches of the highly developed canal 

 system interposed between the laminae ; by their hard perforated 

 Avails ; and by the slit-like aperture (subdivided in Polystomella) 

 situated between the inner margin of the septum and the wall of 

 the previous convolution. 



The structure of Polystomella is described above (pp. 62 et seq.). 

 Nonionina is a simpler form of the same type, characterised by the 

 scantiness of the umbilical deposit, the absence of retral processes, 

 and the fact that the aperture is not subdivided into pores, as in 

 Polystomella, but remains a simple slit. 



Amphistegina is transitional in structure between the Rotalidea 

 and Nummulitidea ; it has simple septa, and the test is not truly 

 symmetrical, the spire being (as in the Rotalidea) slightly helicoid 

 and the aperture on one (the " inferior ") side of the median plane. 

 In the marked development of the alar prolongations it approaches 

 the genus Nummulites. 



In Operculina (Fig. 55) the chambers are simple and disposed in 

 an expanding spiral, some three or four convolutions completing the 

 test. The earlier chambers are produced to a greater or less 

 extent into alar prolongations, and thus a boss-like umbo is formed 

 at the centre ; but the later chambers are simply applied to the 



