CHALK MAKERS, OR FORAMINIFERA. 47 



towards one end. In other forms the chambers 

 approximate so closely that they are flattened at the 

 poles, and united without any intervening isthmus. 

 These may be straight or curved, but still gradually 

 attenuated to one end. From the straight or curved 

 forms the transition is easy to those in which the 

 smallest end is curled into the commencement of a 

 spiral. Then we encounter more 

 perfect spirals, as complete as in 

 the shell of a snail, but always 

 with more or less distinct trans- 

 verse lines, or furrows, indicating FIG S ._ ACATHISTE(;A< 

 the division into chambers. Some- 

 times these spiral shells are nearly smooth, or they may 

 be wrinkled, ribbed, indented, perforated, sculptured in 



FIG. 6. HELIXOSTEGA. 



various ways, spurred, or ,keeled. In some the ter- 

 minal opening is large, in others small. Then to all 



