PROTOZOA. FORAMINIFERA 



19 



which is really the front wall of the preceding chamber; 

 but in the higher vitreous forms each septum (fig. 1, B, b) 

 is formed of two lamellae, owing to the fact that when a 

 new chamber is added to the shell a new wall is secreted 

 next to the front wall of the last chamber. The shell of 

 the vitreous Foraminifera is at first thin, but may after- 

 wards increase in thickness by the addition of material at 

 the surface ; in the higher vitreous forms the outer layers 

 are often traversed by numerous canals and constitute 

 what is known as the supplemental skeleton (fig. 1, B, d). 



A B 



Fig. 2. Dimorphism of Nummulites Icevigatus, Brackleshani Beds 

 (Eocene), Selsea. A, section of the entire shell of the megalospheric 

 form x 9. B, section of the central part of the microspheric 

 form x 9. 



Most of the higher Foraminifera are dimorphic — that 

 is to say, there are two forms of the same species. This 

 fact was first noticed in specimens of Nummulites from the 

 Eocene deposits. In one form, the first or initial chamber, 

 which is seen at the centre when the shell is split, is large 

 and more or less spherical and is called the megalosphere 

 (fig. 2, A); in the other it is much smaller and is known 

 as the microsphere (fig. 2, B). These two forms are found 

 associated together, and were, at one time, described as 

 different species. In the microspheric type the shell 



. 2—2 



