202 FAMILY GLOBIGERINIDA. 



" flush" with that of the cone as a whole ; whilst they are so closely applied to each other 

 that their septal planes become nearly fiat. On the other hand, the depression of the spire 

 may be so great that its superior lateral surface becomes completely flattened ; and in this 

 case it may happen that, if the inferior surface remains flattened, the shell may become almost 

 exactly symmetrical (c), and in extreme cases complanate (f), or even bi-concave (g) ; whilst 

 the segments may be so disposed as to render the inferior lateral surface convex (u), or even 

 conoid (e). Again, the segments may be so ventricose and so loosely attached to each other, 

 that the spire is scarcely closer than that of a Glohigerina ; and there are even cases in which 

 their arrangement approaches that which is characteristic of Textularia. In most of the forms 

 in which the spire is elevated, the chambers of the later convolutions do not extend inwards 

 so as to reach the axis, but leave an "umbilical vestibule," which is sometimes of considerable 

 dimensions (ex. Figs. 91, 102) ; this, however, does not ordinarily communicate (as in Glohlge- 

 rind) with the cavities of the chambers ; and it is frequently filled up by solid shell-substance. 



347. The external aspect of the shell in this genus is often considerably modified by an 

 exogenous deposit of non-porous shell-substance, sometimes translucent, sometimes opaque. 

 Such a deposit very commonly presents itself along the septal bands, converting what would 

 otherwise be fossae into elevated ridges, which may become continuous at their extremities 

 with a similar deposit that forms the outer border of the spire (ex, Figs. 101, 103, lOG, 112) ; 

 in many instances, again, a like deposit forms a sort of columella in the axis of the spire, 

 which shows itself more or less prominently on the inferior lateral surface ; whilst in other 

 cases its presence is chiefly marked by tubercular elevations on the general surface, which are 

 usually more abundant on the lower side than on the upper. The marginal deposit occasionally 

 extends itself, as in CristeUaria, into radiating spire-like prolongations ; but these, in the 

 ordinary Rotalines, are neither large nor numerous. In the ^e.nVi% PlanorbuKna we occasionally 

 find the general surface beset with prominent tubercles, between which the pores are crowded 

 together (Plate XIII, fig. 15). It is in the genus Calcarina, however, that this exogenous 

 deposit is most remarkable (Plate XIV) ; the whole of the spire being invested with an 

 extraordinary thickness of homogeneous shell-substance, which sends out numerous large 

 extensions, and for the nutrition of which an elaborate canal-system is provided. It is not a 

 little singular, however, that this canal-system does not extend to the interior of the spire, 

 which is constructed upon the simplest Rotaline type, and which, although externally masked, 

 is usually very regular. 



348. A very curious feature presented by several modifications of the Rotaline type, is 

 the development of " asterigerine" flaps of shell- substance (Plate XIII, fig. 3), which radiate 

 to a greater or less distance from the umbilical depression on the lower surface, and 

 apparently serve to cover-in secondary lobes of sarcode-substance. These flaps are sometimes 

 so complete, and are disposed so regularly, as to inclose a set of secondary chambers alter- 

 nating with those of the primary spire ; whilst in other cases they are merely rudimentary. 



349. In many forms of the Rotaline series, the typically spiral mode of growth gives 

 place either in an earlier or a later stage of existence to the cyclical. Some approach to this 

 is seen in the flat vermiculate forms of PuIvinuUna ; it is very evident in the advanced condi- 



