450 NATURE AND MAN. 



came to be completely established. This I designated as the 

 &quot;simple&quot; type of Orbitolite structure. 



I was further able to show that these two typical forms were 

 connected by a gradational series of connecting links ; the forma 

 tion of disks of the &quot;complex&quot; type often commencing on a 

 plan resembling that of the &quot; simple ; &quot; and the change from the 

 latter to the former taking place, not at any fixed epoch of growth, 

 but after a variable number of rings had been formed, sometimes 

 abruptly, sometimes more gradually, in the manner to be presently 

 detailed. And I also found that the inner rings of even the largest 

 &quot;complex&quot; disks, if their early growth had taken place on the 

 &quot; simple &quot; type, were not complete, but showed a tendency to 

 one-sided and therefore spiral growth, like that seen in Fig. VI., 3. 



Reflecting on the relations of these highly specialized Forami- 

 niferal types to the simpler forms of the Milioline group, to which 

 (in virtue of the &quot; porcellanous &quot; character of their shells) I 

 referred them, I ventured to construct a hypothetical pedigree; 

 tracing their descent (Fig. V.) from the particle of protoplasm 

 that forms the spheroidal chamber in which every Foraminiferal 

 shell begins, first to an open undivided spiral (i) ; then to a type 

 in which the spire is constricted at intervals (2) ; then to a type in 

 which it is completely divided into chambers by transverse par 

 titions (3) ; then to a type in which the spirally arranged chambers 

 are divided by longitudinal partitions into chamberlets (4) ; then to 

 the &quot;simple&quot; type of orbitolites, in which the spiral plan of growth 

 gives place to the cyclical (5); then to an &quot;intermediate&quot; type, 

 in which the original spiral almost disappears (6) ; and finally 

 to the &quot;complex&quot; type, in which the plan is cyclical from the 

 beginning (7). 



This hypothetical pedigree has found its complete confirmation 

 in a deep-sea Orbitolite of extraordinary delicacy and beauty, which 

 was brought up in the Porcupine Expedition of 1869. For this 

 little disk, about the size of a fourpenny piece, while for the most 

 part truly cyclical, has a long succession of inner chamberlets 

 arranged upon the spiral plan, as in Orlnculina ; these, again, 

 arise from expanded but undivided chambers, like those of a 

 Peneroplis; and these chambers are the continuation of a spiral 

 tube, with occasional constrictions, resembling that of a Spirvlo- 



