STRUCTURE OF NUMMULITB. 



685 



of a series pf chambers, symmetrically disposed in a spiral round 

 a centre, so that a section through its median plane would pre- 

 sent very much the appearance of Fig. 209 ; but each whorl in- 

 vests all the preceding whorls, so as to form a new layer over the 

 entire surface of the disk ; and this layer is usually separated 

 from that which it covers, by an intervening space, which is 

 divided into smaller spaces of more or less regular form, by pro- 

 longations from the partitions that divide the chambers of the 

 central plane. These prolongations are very differently arranged 

 in different species ; thus in some, as Nummulites distans, they 

 keep their own separate course, tending towards the centre ; 

 whilst in others, as N. Icevigata, they inosculate with each 

 other, so as to divide the space that intervenes between one 

 layer and another into an irregular network. Hence in a verti- 

 cal section, such as that of which a part is shown in Fig. 337, we 



Fig. 337. 



Vertical Section of portion of JVttmmK^ites Zffiwi'^ato.* — a, margin of external whorl; 6, one of the 

 outer row of chambers ; c, c, whorl invested by a ; d, one of the chambers of the fourth whorl from 

 the margin; e, e', marginal portions of the enclosed whorls; /, investing portion of outer whorl; g, g, 

 spaces left between the investing portions of successive whorls; 7i, h, sections of the partitions 

 dividing these. 



see not only the succession of chambers along the central plane, 

 each of them having its own roof and floor, and its own lateral par- 

 titions dividing it from other chambers of the same whorl, but 

 we also see the superposition of layers over the inner whorls ; 

 so that any chamber c? in a whorl that is surrounded by three 

 others, is shut in above and below, not only by its proper shelly 

 covering, but by three additional layers formed by the pro- 

 longation of the shelly investments of the external whorls ; and 

 in like manner, the innermost of the chambers here represented 

 (that nearest e) is enclosed by nine layers above and below, in 

 addition to that by which it is itself covered, these nine layers 

 being extensions of the covering of the nine whorls that surround 

 it. S"otwithstanding that the inner chambers are thus so deeply 

 buried in the mass of investing whorls, yet there is evidence that 

 the segments of sarcode which they contained, were not cut off 

 from communication with the exterior ; but that they may have 

 retained their vitality to the last. The shell itself is almost every- 

 where minutely porous, being penetrated by parallel tubuli, 



