42 



THE FORAMINIFEEA 



GrROMiiDyE IiavG 110 calcareous shell, but are chitinous 

 or horii}^ in texture. This particular kind of invest- 

 ment is also found in a few forms in the genus 

 Miliolina and in some usually arenaceous genera of 

 the family Textulaeiid.e. 



Descriptive Terminology. 



Before proceeding to discuss the subject of the 

 various methods of growth in the foraminiferal shell 



SEPTAL LINZS 



; PERIPHERAL 

 MARGIN 



UMBILICUS ' 



PERIPHERAL 

 MARGIN 



SEPTAL FACE 

 --APERTURE 



.SEPTAL LINES 



•spiral suture 

 Fig. 25. — An Involute Nautiloid Foraminifee, Nonionina depeessula, 



TO ILLUSTEATE THE ExTEENAL PaRTS OF THE ShELL. 



it will be- well to give a few of the terms used in 

 describing the parts of the shell and their relations 

 to one another. (See fig. 25.) 



The outward indication of the septum is usually 

 either a ridge or depression — the septal line ; some- 

 times the suture is flush with the shell surface, and is 

 then only recognised by a difference in its trans- 

 lucency. 



The superficial area of the septum is called the 

 seijtal p)lcnie or septal face. 



