DESIGN IN THE ORGANIC WORLD. 



449 



and their openings at the margin forming but a single row of 

 pores (Fig. IV., i, 3, 4). The arrangement of the first-formed 

 chamberlets, moreover, presented a single departure from the 

 cyclical plan, showing a distinctly ^/"/V^/ disposition (Fig. IV., 2); 

 the mouth of the spire, however, rapidly opening out by successive 



Fig. IV. 

 Disk of Simple Type of OrbitoUtc. 



1. Surface of Disk, showing later growth of concentric rings of chamberlets 

 around a first-formed spire. 



2. Central portion enlarged. 



3. Edge of Disk, showing single row of marginal pores. 



4. Vertical Section, showing succession of chamJjerlets communicating 

 with each other radially by passages in the annular partitions, and laterally by 

 the annular canals, whose sections are seen as dark spots. 



additions, so as to enclose the " nucleus " ; after which all suc- 

 ceeding additions were complete rings, so that the cyclical plan 



