232 



THE PROTOZOA 



just before. Hence a distinction must be drawn between mono- 

 thalamous or single-chambered shells and polythalamous shells, 

 made up of many chambers formed successively. In the latter 

 type the new chambers may be joined in various ways to the old, 



2.Lagena 



4.Frondicularia O.CIobigenna 



O.Planorbulino 10 n.Nummulites 



Fia. 93. Shells of various genera of Foraminifera. In 3, 4, and 5, a shows the 

 surface-view, and b a section ; Sa is a diagram of a coiled shell without supple- 

 mental skeleton ; 85, of a similar form with supplemental skeleton (.i.tk. ) ; 

 10, of a form with overlapping whorls ; in lla half the shell is shown in hori- 

 zontal section ; b is a vertical section. In all the figures a marks the aperture 

 of the shell ; 1 to 15, the successive chambers, 1 being always the oldest or 

 initial chamber. From Parker and Haswell. 



producing usually either a linear or a spiral series, and the utmost 

 variety of shape and pattern results in different species (Fig. 93). 

 Some polythalamous species exhibit a peculiar dimorphism (Fig. 94) 



