102 THE FORAMINIFERA 



single apertures. As the spirat increases in width the chambers 

 become divided into chamberlets, and the number of apertures is 

 correspondingly increased, the arrangement at this stage repeating 

 that which in the earlier stages of growth is common to all 

 varieties of Orbiculina,, except that there are no alar prolonga- 

 tions. When the stage of the spiral 

 mode of growth is complete, the 

 chambers become successively more 

 and more embracing and the annular 

 arrangement is attained. 



In the megalospheric form (Figs. 

 33, A, and 34) the megalosphere is 

 pear-shaped. A spiral passage leads 

 from it, and extends round about 

 three-quarters of the circumference 

 of the megalosphere. As in the case 

 of Peneroplis and Orbiculina the walls 

 of the megalosphere and the spiral 

 Fl - 34 - passage may be perforated (Fig. 34). 



Central chambers of 0. riiarginalis. mi. i i- i- i_-Yj>n 



showing the perforated wall of the J-he single chambers which follow 

 megalosphere. are usua iiy on ]y three or four in 



number, and beyond, the chambers become subdivided, and the 

 arrangement resembles that of the microspheric form. 



The dimensions of the central chambers in the specimens 

 which I have examined are as follows. It will be seen that as in 

 Peneroplis those of the megalosphere vary in the samples from 

 different localities. 



No. of Highest Lowest Average 



Specimens Value of Value of Value of 



examined. M or m. M or m. M or m. 

 Megalospheric 



from Aripo (Ceylon) . 43 53 M 24 M 36 ^ 



W. Indies . 47 78 /* 37 p 51 A* 

 Microspheric 



from Aripo . 17 19 /t 15 n 17 /t 



,, W. Indies . 1 ... ... 18 M 



In Orbitolites duplex, Carpenter, the arrangement is at first 

 sight similar, but the chamberlets are more elongated in a direc- 

 tion perpendicular to the face of the disc. Here, again, they are 

 in communication by a single annular canal, but the apertures 

 which open out of it and lead to the chamberlets of the succeeding 

 annulus are disposed obliquely and lie in two planes, one on 

 either side of the median plane (cp. Fig. 38, d). There are thus 

 in typical specimens two rows of pores at the margin of the 

 disc. There is, moreover, a difference in the shape and arrange- 

 ment of the chamberlets. Instead of the regularly curved series 

 of quadrangular chamberlets which make up the well-marked 

 annuli of 0. marginalis, the chambers (especially those near the 



