FORAMINIFERA 



75 



The shells of this group are typically many-chambered and cal- 

 careous, but a fair number are one-chambered, and most of these and 

 some of the many-chambered shells are composed of foreign particles 

 {arenaceous). Either kind may be imperforate or perforate by numerous 

 small pores, but most of the non-calcareous shells are imperforate. 

 The one-chambered shells are of various shapes. They usually grow 



A B 



Fig. 62. A, Section of a foraminifer in which each septum is formed of a 

 single lamella. B, One in which the septum is formed of two lamellae and 

 a supplemental layer is present. After Carpenter, a, passages between the 

 chambers; h, septum; c, anterior wall of last chamber; d, supplemental 

 skeleton. 



A B 



Fig. 63 

 Selsea. 

 form, 



Dimorphism of Numniulites laevigatus, Bracklesham Beds (Eocene), 

 From Woods. A, Section of the entire shell of the megalospheric 

 X 9. B, Section of the central part of the microspheric form, x 9. 



by extension at their openings. Shells with more than one chamber 

 grow by the addition of chambers. The protoplasm bulges from the 

 mouth of the shell and there secretes around itself a new chamber into 

 which opens the previous mouth. The chambers may be arranged in 

 a straight line, as in Nodosaria (Fig. 6B), or in a spiral, as in Poly- 

 stomella, etc. (Figs. 62, 63, 65), or occasionally irregularly; and the 

 shell may be strengthened by the deposition, upon their original 



