SIMPLE TYPE OF ORBITOLITES. 



493 



on either side of it in the same Ring ; so that each circular Zone of 

 chamberlets might be described as a continuous annular passage, 

 dilated into cavities at inteiwals. On the other hand, each Zone 

 communicates with the zones that are internal and external to it, 



Fig. 250. 



Simple disk of Orbitolites complanatus, laid open to show its 

 interior structure : — a, central chamber ; b, circumambient 

 chamber, surrounded by concentric zones of chamberlets, con- 

 nected with each other by annular and radiating passages. 



by means of passages in a radiating direction ; these passages run, 

 however, not from the Chamberlets of the inner zone to those of the 

 outer, but from the connecting Passages of the former to the Cham- 

 berlets of the latter ; so that the Chamberlets of each Zone alternate 

 in position with those of the zones internal and external to it. The 

 Radial passages from the outermost annulus make their way at once 

 to the margin, where they terminate, forming the ' pores ' which (as 

 already mentioned) are to be seen on its exterior. The central 

 Nucleus, when rendered sufficiently transparent by the means just 

 adverted-to, is found to consist of a Primordial Chamber (a), 

 usually somewhat pear-shaped, that communicates by a narrow 

 passage with a much larger Circumambient Chamber (6), which 

 nearly surrounds it, and which sends-off a variable number of 

 radiating passages towards the Chamberlets of the first Zone, which 

 forms a complete ring around the circumambient chamber.* 



* Although the above may be considered the typical form of the Orbi- 

 tolite, yet, in a very large proportion of specimens, the first few Zones 

 are not complete circles, the early growth having taken-place rather in a 



