46 OF THE FORAMINIFERA GENERALLY: 



shells that bear them. In shells of this type, however, which have been long dead and exposed 

 to the action of sea- water, the vitreous transparence often gives place to a lustrous white 

 opacity, that is particularly striking in the prominent tubercles ; as is remarkably shown in the 

 tuberculated variety of Plamrhdina vulgaris (Plate XIII, fig. 1 5) . The texture of the shells of 

 this type is much firmer tl;an that of the porcellanous shells ; approaching closely to that of the 

 inferior forms of dentine, or to that of the terminal portion of the crab's claw. 



59. Between the comparatively large apertures which are common in the Rotaline type, 

 and the minute tubuli of the OpcrcnVine, there is such a continuous gradation as indicates that 

 their mode of formation, and probably their uses, are essentially the same. In the former it 

 has been demonstrated by actual observation that they allow the passage of pseudopodial 

 extensions of the sarcode-body through every part of the external wall of the chambers 

 occupied by it ; and there is nothing to oppose the idea that they answer the same purpose 

 in the latter, since, minute as they are, their diameter is not too small to enable them to be 

 traversed by the finest of the threads into which the branching pseudopodia of Foraminifera 

 are known to subdivide themselves. And it seems the more likely that they answer this 

 purpose, when attention is given to the remarkable continuity which they exhibit through a 

 considerable thickness of shell, formed of numerous lamellffi that seem to have been added at 

 successive periods of growth (Plate XIX, fig. 3). Now, if this be the case, it is not difficult to 

 account for the production of a texture closely resembling that of dentine, without having 

 recourse to the hypothesis (xxv, p. 421) of a higher organization in the bodies of 

 these animals than that which we have other grounds for admitting. For if the 

 shell-substance be, as there seems reason to believe, an excretion from the |3rotoplasmic 

 mass of which the body itself is composed, each new lamella, as it is added to the pre- 

 ceding, will mould itself upon the pseudopodia that issue from the orifices of the subjacent 

 surface ; either some difference in their composition, or the activity of the changes con- 

 tinually taking place in their substance (^ 34), preventing them from being involved 

 in the consolidation which takes place ai'ound them. We have an illustration of the same 

 kind on a larger scale, in the extension of the straight branches of the canal-system which 

 pass through the solid masses of shell-substance that occupy the umbilical regions in the shells 

 of Calcdrina and Fohjdomella (Plate XIV, fig. 3, and Plate XYI, fig. 3) ; the successive accretions 

 of vitreous material being so disposed around the extensions of the sarcode-body which occupy 

 these passages, as to preserve their continuity throughout, and thus to maintain the most 

 direct relation between the parts of the canal-system most deeply buried beneath these accre- 

 tions, and the external surface from which they become further and further removed. If this 

 should prove to be the true account of the formation of the dentine-like tissue produced by 

 the higher type of Foraminifera, it will deserve consideration whether a like explanation may 

 not be applicable to the formation of analogous calcified tubular tissues in animals much more 

 elevated in the scale ; especially since there is increasing reason to believe that the develop- 

 ment of such tissues takes place after a far simpler fashion than has been commonly 

 supposed, and that their foundation is laid in a homogeneous blastema, rather than in a 

 matrix possessing distinct organization. 



60. Besides these two principal types of shell-structure, another is met with in certain 

 groups of Foraminifera, which is designated as the arenaceous; the shells being formed, either 



