THE SHELL STRUCTURE 



37 



are very fine ; they are about '01 niiii. in their 

 transverse section (tig-. '22, b). Certain parts of the 

 shell are not tubulated in the hyaline or perforate 

 group, and these layers may be regarded as an 

 exogenous growth. This secondary structure is 

 found only in the higher types of foraniiniferal 

 shells, and chiefly in those which have a double sep- 

 tum — that is, a complete and separate investment 

 to each segment, as in Botalia (fig. 23). It is seen 



int. o. 





ex. 











Fig. 23. — Median Section of Kotalia Beccarii, showing Interseptal 

 Canals (int. c), Exogenous Shell-layer (ex.), and Tubuli 

 (t.). X 80. Oriijiniil. 



on the external part of the shell, tilling up sutural 

 lines and umbilical depressions, and even forming 

 a thick superficial coating to the shell or internal 

 j^illars, thereby adding greatly to the strength of 

 the shell-wall. This layer often imparts an orna- 

 mental aspect to the surface of the shell in the form 

 of beads or cones with the apices directed inwards, 

 and the convex bases exposed, as in ('ijcloclypen,^ 

 (fig. 24) ; or it may take the form of spurs and 



