96 W. B. CARPENTER ON THE STRUCTURE OF ORBITOLITE8. 



each ring being connected with the next by two rows of 

 radial stolons, instead of one. Now, seeing that the most highly 

 developed form often begins life in the simplest form, then passes 

 through the intermediate form, and then undergoes this separa- 

 tion of the superficial plane by an intermediate plane — the next 

 stage being that the annular canal is split (as it were) into two — 

 I came to the conclusion that there was no actual specific distinc- 

 tion between the simple and the complex types, but that they were 

 merely stages of development of the same organism, which in 

 tropical seas undergoes a higher development than in colder 

 regions. 



Having investigated this subject very carefully, I made it, in 

 1856, the basis of a disquisition on the Range of Variation of 



Fig. 8. 

 Disk of Simple Type of Orbitolite (0. marginalis). 



1. Surface of disk, showing later growth of concentric rings of chamber- 

 lets 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 chamberlets communicating 

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

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



