164 THE FOEAMINIFERA 



which exhibit a succession of segments joined together 

 in a regular manner. There are modifications of these 

 fundamental types in which two or even three plans 

 of growth are seen (bimorphous and trimorphous 

 forms). 



The larger species are arenaceous^ the smaller 

 ones usually hyaline, and conspicuously perforate. 



The sub-family Buliminin^ includes types of 

 shell-structure which may be either triserial or 

 biserial, but spirally wound round the axis of growth. 

 The attenuated and feebly developed species are 

 usually biserial in arrangement, sometimes twisted, 

 sometimes not, but differing in other points when 

 compared with the typical textularid shell, as, for 

 example, in the position and shape of the aperture. 



The most complex of the textularid forms are 

 grouped together in the next sub-family, the 

 Cassidulinin^. Here the plan of growth may be 

 explained by imagining a Textidaiia coiled upon 

 itself, as in Gassiclulina. A modification of this 

 plan is seen in EJirenhergirta, in which the shell is 

 partly unrolled, as it were ; and consequently shows 

 a convex dorsal surface and a concave side contain- 

 ing the early spiral portion. The test in this group 

 in all cases is hyaline, and usually surface-polished. 



Suh-family 1. Textulaeiin.e. 



Test having a biserial or triserial arrangement of 

 chambers. Sometimes bimorphous or even tri- 

 morphous. 



