220 FAMILY GLOBIGERINIDA. 



shown in fig. 4 ; this section, being taken in such a plane as to cut through the outer whorls 

 <•/*, (f, a", passes entirely over the surface of the two inner whorls a^ and a. The disposition of 

 the chambers, as indicated by such sections, is ideally shown in fig. 8. The turns of the 

 spire are separated from each other by the interposition of a thick layer of solid shell-sul)stance; 

 and this is quite distinct from the proper walls of the chambers, as is well seen in fig. 4, 

 where the walls, b, of the newest chambers are shown to be entirely destitute of any such 

 addition, whilst in the preceding part of the same whorl we observe them encrusted by a thin 

 additional layer d, and as we trace this layer backwards to d^ and r/", we perceive that it pro- 

 gressively augments in thickness, until it acquires its maximum at d^, just where it is covered 

 by the subsequent whorl. This distinction between the proper walls of the chambers and 

 the " supplemental skeleton" can be traced to the very centre of the spire. The septa are 

 entirely formed by the infolding of the proper walls of the chambers, which at their planes of 

 junction are destitute of the pores with which their substance is elsewhere minutely 

 penetrated ; sometimes the septum consists throughout of only a single lamella, as in Discor- 

 bina ; but most commonly it is composed at its outer part of two lamelte, which, though 

 usually in contact, sometimes diverge to give passage to canals ; whilst sometimes it is double 

 throughout. There does not appear, however, to be any regular " interseptal system" as in 

 Operculina and Polysiomella. The communication between the adjacent chambers of the same 

 whorl is efi'ected, as in Polj/sfomella, through a series of pores (fig. 3, c) disposed at pretty 

 regular intervals along the inner margin of the septum ; these may be considered as formed 

 by the bi'idging over of an elongated fissure by delicate bars of shell-substance. The 

 external aperture of course presents the same character (fig. 8, c). 



383. That the spines entirely originate from, and are strictly appendages of, the 

 " supplemental skeleton" is well seen in fig. 4, which shows their connexion with its 

 successive convolutions. Thus the spine / is one of the oldest, being traceable inwards to 

 the earlier whorls ; whilst those marked /\ /", /'\ f\f^ are obviously of progressively later 

 production, their respective origins being further and further removed from the centre of 

 the spire. It is, moreover, to be observed that each spine receives an augmentation in 

 thickness as the convolution from which it sprang is encircled by others ; this augmen- 

 tation, however, is not marked (as in the spines of Echini) by lines of demarcation between 

 the earlier and the later formations ; and there is every reason to believe that the 

 growth of the spines, both in length and in diameter, is continuous rather than interrupted. 

 Although it might seem, from the examination of such sections only as are taken in the 

 direction of the equatorial plane or in one parallel to it, as if the course of the spire must be 

 seriously interrupted by the radiation of the spines (which sometimes appear to be so 

 interposed between consecutive chambers as completely to separate them), yet the fact is that 

 owing to the turbinoid form of the spire, a spine projecting from an earlier whorl is very 

 little in the way of even the next convolution ; for as this passes by the spine on a difi'erent 

 level, its chambers are but slightly encroached-on, and this only upon the side which looks 

 towards the apex of the spire, — as will be readily understood by examining the relation of 

 tiie last half-convolution, visible in such a specimen as the one delineated in Plate XIV, fig. 1, 

 to the pre-formed spines, or by an inspection of the ideal represented in fig. 8, 



