GENUS TEXTULARIA. 191 



In the Textdarim whose segments are most globose, the crescentic aperture may increase in 

 breadth so as to become semilunar or even gibbous ; whilst in those whose segments are most 

 depressed it may be narrowed to a mere sht. 



318. The proper substance of the shell is hyaline, with large pores usually not very 

 closely set, though in some varieties more minute and more nearly approximated ; it is occa- 

 sionally to be observed that the pores open at the surface into deep hexagonal pits. It very 

 commonly happens, however, that the shell, as it increases in size, becomes incrusted \v'ith 

 arenaceous particles, which are commonly large and coarse, and which may entirely conceal 

 the pores from superficial observation ; and in some of the smallest examples from deep 

 water, the shell-substance appears externally to be almost as completely replaced by an aggre- 

 gation of fine sand-grains as it is in Lituola or Valmlina. In the coarsest TextMlaria, however, 

 I have never failed to bring the pores distinctly into view by examining the shell by trans- 

 mitted light when rendered sufficiently transparent by the removal of the wall on one side ; 

 and in the smallest and finest I have been able to distinguish the pores in the interspaces 

 between the sand-grains, when I have made use of a sufficient magnifying power. — It is in 

 some of the large fossil Textularice that the arenaceous incrustation is the coarsest in its 

 texture, so as to give the greatest roughness to the surface. 



319. Internal Structure. — When the shell is laid open by section, we find that its struc- 

 ture is of the simple type that might be expected from its external configuration. Each 

 segment is so applied to those which preceded it, that its inner wall is partly formed by the 

 inner wall of the alternate segment which it follows, whilst its posterior wall is formed by the 

 anterior wall of the penultimate segment of its own series. Thus the septal lamella is always 

 single ; and there is no differentiation between its texture and that of the rest of the chamber- 

 wall. The apertural passages by which each chamber communicates with that which precedes 

 and with that which follows it, are very near to each other ; as is well shown in Plate XII, 

 fig. 14, which represents the body of the animal and the membranous basis of the shell after 

 the removal of its calcareous substance by dilute acid, and in Plate XXII, fig. 9, which 

 represents a siliceous cast of the body of a Textularia from the Greensand. It thus becomes 

 obvious that the plan of growth depends upon the oblique gemmation of the successive seg- 

 ments of the animal body, alternately towards one side and then towards the other; and it is 

 very easy to see how such a bi-serial arrangement may become either uni-serial or tri-serial. 

 — In some of the largest fossil Textularice, the principal cavities of the chambers are irregu- 

 larly subdivided by secondary partitions, as in Lituola. 



320. Suh-generic Mollifications. — The genus Bi(/erina of D'Orbigny is nothing else than 

 a Textularia which passes from the bi-serial to the uni-serial plan ; the alternating arrangement 

 of the segments giving place to a single rectilineal succession resembling that of Nodosaria ; 

 and the aperture becoming central and circular, sometimes with a lip that may even be slightly 

 elongated outwards into a tube. A like change in the plan of growth, the aperture remaining 

 marginal, gives rise to the form distinguished by D'Orbigny as Gemmulina. In the Gaudnjina 

 of D'Orbigny, again, the arrangement of the chambers is at first tri-serial, as in the typical 

 Valvulina (f 220) ; but in the course of growth, the tri-serial alternation gives place to the 



