THE LAURENTIAN AND HURONIAN PERIODS. 73 



together; and it may consist of a single chamber (fig. 26, a), 

 or of many chambers arranged in difterent ways (fig. 26, b-f). 



Fig. 26. — Shells of living Fora7nitiifera. a, Orbulhia nniversa, in its perfect condi- 

 tion, showing the tubular spines which radiate from the surface of the shell ; b, Globi- 

 gerina bulloides, in its ordinary condition, the thin hollow spines which are attached to 

 the shell when perfect having been broken off ; c, Texhilaria variabilis ; d, Peiieroplis 

 planahis ', e, Rotalia coficajnerata ; f, Cristellaria siibarctiatiila. [Fig. a is after 

 Wyville Thomson ; the others are after Williamson. All the figures are greatly en- 

 larged.] 



Sometimes the shell has but one large opening into it — the 

 mouth ; and then it is from this aperture that the animal pro- 

 trudes the delicate net of filaments with which it seeks its 

 food. In other cases the entire shell is perforated with 

 minute pores (fig. 26, <?), through which the soft body-substance 

 gains the exterior, covering the whole shell with a gelatinous 

 film of animal matter, from which filaments can be emitted at 

 any point. When the shell consists of many chambers, all of 

 these are placed in direct communication with one another, 

 and the actual substance of the shell is often traversed by 

 minute canals filled with living matter {e.g., in Calcari7ia and 

 Nummulina). The shell, therefore, may be regarded, in such 

 cases, as a more or less completely porous calcareous structure, 



