A. Helicoidea = \ 



48 THE FORAMINIFEEA 



2. POLYTHALAMIA. 



/Cyclostega, D'Orb. 



Helicostega, D'Orb. 



Entomostega, D'Orb. 



Enallostega, D'Orb. 

 ^Agathistega, D'Orb. 



B. Bliahdoidea = Stichostega, D'Orb. 



c. Soroidea. Contains the genus Acerrulina, 



Schultze (= Gjfpsina). 



The above classificatory schemes, based entirely 

 on external form, do not allow for those types of 

 Foraminifera which change their plan of growth once 

 or even twice during their development. For ex- 

 ample, Spiroplecta is at first of a nautiloid-spiral 

 form, then biserial, and afterwards uniserial, thus 

 possessing characters w4iich belong to orders 4, G, and 

 3 of D'Orbigny's classification, in succession. Or 

 Hauerina, which is at first milioline and then piano- 

 spiral. Such complex forms are spoken of as dimor- 

 jyhoiis and triniorphoiis} 



IsomoTphis7n in the Foraiiiinifera. — When two or 

 more forms distinct in shell structure exhibit similar 

 external outlines they are said to be isomorphous. 



As examples among the Genei-a of Foraminifera 

 the following may be quoted : — 



^ These terms should not be confused with • dimorphic ' and * dimor- 

 phism,' wliich have a totally different meaning 



