THE FORAMINIFERA 



105 



at the periphery open to the exterior by vertical rows of pores 

 at the margin of the disc. 1 



The Microspheric Form. The centre of the disc of this form 

 is much thinner than that of the megalospheric (Fig. 36, a and b). 

 It is often the seat of secondary growth which occurs towards the 

 end of the vegetative phase, giving rise to a button -like ex- 

 crescence and accompanied by absorption of the original central 



FIG. 37. 



Orbitdites complanata, Lamk. Central 

 regions of sections of the megalospheric 

 (A) and microspheric (B) forms, in the 

 median plane of the discs, x 100. ca.ch, 

 circumambient chamber ; M, megalo- 

 sphere ; p, partition ; sp.p, spiral passage ; 

 11, the last undivided chamber of the 

 microspheric form. 



chambers. If this has not occurred an arrangement similar to that 

 of the central regions of the microspheric forms of marginalis and 

 duplex is revealed by section. In two specimens I find that the 

 microsphere has a mean diameter of 17 and 18 /x, a spiral 

 passage is absent, and seven to eleven single chambers succeed 

 the microsphere. These are followed by subdivided chambers, 

 continuing the spiral, and the mode of growth then changes to 

 the cyclical as in the other species (Fig. 37, B). 



In some varieties, at least, of this species the microspheric form 

 attains a much larger size than the megalospheric (Fig. 36, A 

 and B), and the large forms with double and contorted margins, 

 described as variety laciniata, Brady, are all, as far as my experience 

 goe.s, microspheric. It seems, indeed, that the peculiarity of the 

 margin of this form may be regarded as a provision for supplying 

 a larger number of peripheral brood chambers for the accommoda- 

 tion of the megalospheric young into which the protoplasm 

 becomes divided. 



1 For the details of the structure, cp. Carpenter's descriptions (8 and 9). 



