16 PEOTOZOA. FORAMINIFERA. 



homogeneous, and is not perforated by tubes, the whole 

 of the pseudopodia passing out by one or two large 

 apertures. Sometimes, however, the surface of the shell 

 is pitted, producing at first sight the appearance of per- 

 foration. 



In the arenaceous forms the shell consists of grains of 

 sand united together by a cement, the appearance of 

 the shell varying according to the material of which the 

 grains are composed. The cement may be formed of 

 chitinous, or of calcareous and ferruginous material. Usu- 

 ally the test is imperforate. 



The chitinous forms do not occur as fossils. In one 

 genus only is the test siliceous. 



With regard to the form of the shell, in some genera 

 it consists of a single chamber, when it is said to be 

 unilocular, as in Orbulina, in which it is spherical, and in 

 Lagena (fig. 2 F), in which it is generally flask-shaped. 

 In other cases it consists of several chambers, which are 

 usually placed in communication with one another by 

 means of perforations in the walls (septa) between them*. 

 The arrangement of the chambers in these multilocular 

 forms is very varied, they may occur in a straight line as 

 in Nodosaria (fig. 2 H), in a curved line as in Dentalina, 

 in a plane spiral as in Cristellaria (fig. 2 G), or in a heli- 

 coid spiral as in Rotalia (fig. 2 L, M}. In some spiral 

 forms the earlier whorls are embraced partly or entirely 

 by the later ones, so that sometimes the last whorl only is 

 visible (e.g. Cristellaria); but when the later chambers 

 are merely attached to the extremities of the earlier ones, 

 all the whorls are visible (e.g. Rotalia). 



* The communication in Nodosaria and its allies is through the 

 mouth-aperture of the chambers which open into one another, and only 

 to a small extent through perforations in the walls. 



