THE SHELL STEUCTUEE 43 



In the nautiloid forms the entire shell has two 

 lateral faces and ?i i^erlplieral mm-gin. 



The septal plane has three angles^ a i^eriplieral 

 and two iimhilical angles. 



Each segment has three Jitargins, a posterior 

 (nearest the primordial chamber), an anterior (nearest 

 the oral aperture), and n periplieral. 



In trochoid shells the surface on which the 

 primordial segment appears is termed the posterior or 

 siqjerior surface^ whilst the opposite, usually flat side, 

 is the inferior surface. 



In the nodosarian type the lateral aspect is that 

 seen when the shell is viewed leno'thwise ; the oral 

 or (Ulterior aspect being that seen when the ultimate 

 septal face is presented to view. 



In compressed forms, as Lingulina, we speak of 

 the edge of the shell as li^peripjhero-lateral aspect. 



The two ends of the rectilineal shell are referred 

 to as the aboral (= proximal) extremity, and the oral 

 (= distal) extremity. 



Plans of Geowth. 



We have already seen that certain of the 

 Foraminifera, as Lagena and Gromia, are single- 

 chambered, which is the result of a complete 

 separation of the segments of sarcode, each having 

 a distinct shell. AYhen, however, as is usually the 

 case, the sarcode body consists of man}' connected 

 segments, these are arranged according to a definite 

 plan or tendency inherent in the primordial and 



