502 FORAMTNIFERA '. TEXTULARIA J BULIMINA J ROTALIA. 



(after the leading forms of each) as the , Textularian and the 

 Rotalian. For notwithstanding the marked difference in then- 

 respective plans of growth, the characters of the individual cham- 

 bers are the same ; their walls being coarsely-porous, and their 

 apertures being oval, semi-oval, or crescent-shaped, sometimes 

 merely fissured. In T extulari a (Plate xv., fig. 14) the chambers 

 are arranged biserially along a straight axis, the position of 

 those on the two sides of it being alternate, and each chamber 

 opening into those above and below it on the opposite side by 

 a narrow fissure ; as is well shown in such ' internal casts' 

 (Fig. 253, a) as exhibit the forms and connections of the segments 

 of Sarcode by which the chambers are occupied during life. In the 



Fig. 253. 



Internal siliceous casts, representing the forms of the segments 

 of the animals of a, Textularia, B, Rotalia. 



genus Bidimina the chambers are so arranged as to form a spire 

 like that of a Bulimus, and the aperture is a curved fissure whose 

 direction is nearly transverse to that of the fissure of Textularia ; 

 but in this, as in the preceding type, there is an extraordinary 

 variety in the disposition of the chambers. In both, moreover, 

 the Shell is often covered by a sandy incrustation, so that its 

 perforations are completely hidden, and can only be made visible 

 by the removal of the adherent crust. 



386. In the Rotalian series, the chambers are disposed in a Tur- 

 binoid spire, opening one into another by an aperture situated on the 

 lower and inner side of the spire, as shown in Plate xv. , fig. 18 ; the 

 forms and connections of the segments of their Sarcode-bodies being 

 shown in such ' internal casts ' as are represented in Fig. 253, b. 



