C— GEOLOGY 69 



that with the passage of time there was a progressive reduction in the 

 number of heterosteginal chambers. This was established by counting 

 the number of septa between the chambers and treating the figures for a 

 large number of specimens statistically. In the lowest part of the section 

 the range in number was 30-18, with a major peak at 24. At the top of 

 the section the range was 30-16, with the major peak at 21. The amount 

 of reduction is not great, but that it took place was indisputable. When, 

 however, material that has been collected from a series of isolated ex- 

 posures representing a much longer range of time is examined the evidence 

 for progressive reduction is much more striking. Workers on Tertiary 

 deposits, when correlating one exposure with another, find that the 

 evidence afforded by counting the chambers in these foraminifera is in 

 complete accord with that derived from stratigraphical and faunal sources. 

 The reduction indicated above is found to progress steadily from the 

 base of the Oligocene, where the range is 38-21 with a maximum number 

 of specimens having from 32-27, to the opening of the Quaternary, where 

 the range is only 4-2 with a maximum at 3. 



In this thoroughly well established evolutionary series we find that the 

 relationship between Development and Evolution closely accords with 

 that already seen in Zaphrentis. In the development of the earlier species 

 of Cycloclypeus both operculine and heterostegine stages are well repre- 

 sented. In later species the operculine stage disappears and the hetero- 

 stegine undergoes the progressive • reduction described above. Here 

 also the characteristic cycloidal chamber of Cycloclypeus appears first in 

 late life and, with the passage of the generations, shifts back to earlier 

 and yet earlier stages of growth until the heterostegine stage has almost 

 completely disappeared also. In this example the principle of adult 

 recapitulation with its accompanying phenomena of tachygenesis and 

 lipopalingenesis dominates an even larger proportion of the life-history 

 than it did in Zaphrentis, for apparently only the proloculum is unaffected 

 by it. 



Lack of time and space forbids a detailed consideration of other examples 

 (cp. Cumings, and George), but lest it should be thought that the above 

 are exceptional a number of others must be briefly mentioned. Among 

 the lamellibranchs is the case of the gens Gryphcea incurva so well estab- 

 lished by Trueman (1915). Here the earliest known developmental 

 stage, the prodissoconch, is a minute embryonic bivalve shell identical 

 with that of the oyster and of other lamellibranchs. This furnishes a 

 clear case of juvenile recapitulation. The neanic phases of development 

 are well known for all the members of the gens from Ostrea irregulare to 

 Gryphcea incurva. In each case the resemblance of the young of later 

 forms to the adults of the earlier forms is most striking. Here, however, 

 lipopalingenesis plays no actual part. The same is true also for the gens 

 Inoceramus concentricus-sulcatus established by Woods. 



For the gastropods. Smith (1906) has worked out the evolution of 

 Volutilithes sayana through V. petrosus from V. limnopsis. His material 

 was abundant and its time succession based upon a series of good geological 

 exposures. He shows that in the development of V. limnopsis the surface 

 of the shell is at first quite smooth. It then becomes decorated for a 



