68 



SECTIONAL ADDRESSES 



of animal organisms. Unfortunately the number of cases in which the 

 evidence is as satisfactorily established from the standpoints of quantity, 

 development, variation and stratigraphical precision as for the example 

 we have studied in such detail above is relatively small. 



There is no group of organisms for which systematists have made such 

 a full use of the principle of adult recapitulation as in the lowliest forms of 

 life preserved as fossils, viz. the foraminifera. One illustrative example 

 may be taken from the Orbitoides group. It includes the genera 

 Operculina, which first appeared in the Cretaceous ; Heterostegina, in the 

 Eocene ; and Cycloclypeus, in the Oligocene. In Operculina the shell is 

 coiled spirally in a single plane and is divided by septa into chambers. 



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Fig. 2. — Diagram showing the structure and development of Cycloclypeus post- 

 indopacificus . Protoconch (black spot) . Operculina and Heterostegina stages 

 (thick outline) . Cycloclypeus stage (thin outline) . (Modified from Tan Sin Hok.) 



As the height of the coils increases with growth the chambers become 

 tall and narrow. In Heterostegina the early development repeats in a 

 typical manner the condition exhibited by Operculina, but in the outer or 

 later coils the height increases greatly and the chambers become corre- 

 spondingly much taller. Owing to an increasing forward bend in the 

 middle the latter become practically semicircular, and at the same time 

 each is divided into a series of chamberlets by the formation of walls 

 across the chamber. In lowly species of Cycloclypeus (Fig. 2) the inner- 

 most coils are again typically operculine and are followed by others which 

 are similarly heterostegine. In later development the height of the coils 

 continues to increase until the chambers are quite circular. Henceforth 

 the shell grows in size by the formation of successive chambers added to 

 the outer margin of the shell. Tan Sin Hok, working upon material 

 collected carefully from a series of strata in a continuous exposure, showed 



