THE LAURENTIAN AND HURONIAN PERIODS. 73 
together ; and it may consist of a single chamber (fig. 26, a), 
or of many chambers arranged in different ways (fig. 26, -/). 
Fig. 26.—Shells of living Poramintfera. a, Orbulina universa, in its perfect condi- 
tion, showing the tubular spines which radiate from the surface of the shell; 4, Glod7- 
gerina bulloides, in its ordinary condition, the thin hollow spines which are attached to 
the shell when perfect having been broken off; c, Textudlaria variabilis; d, Peneroplis 
planatus; e, Rotalia concamerata; f, Cristellaria subarcuatula. (Fig. a@ is after 
baa Thomson; the others are after Williamson. All the figures are greatly en- 
arged.] 
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, e), 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 (¢g., in Calarina and 
Nummulina). ‘The shell, therefore, may be regarded, in such 
cases, asa more or less completely porous calcareous structure, 
