FORAMINIFERA AND RADIOLARIA. 



89 



into those above and below it on the opposite side by a narrow fissure; as 

 is well shown in such ' internal casts ' (Fig. 328, A) as exhibit the forms 

 and connections of the segments of sarcode by which the chambers were 

 occupied during life. In the genus Bulimina 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 extraordi- 

 nary 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. And so many cases are now known, in which the shell 

 of TextularincB is entirely replaced by a sandy test, that some Systematists 

 prefer to range this group among the A renacea. 



483. In the Rotalian series, the chambers are disposed in a turbinoid 

 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 con- 

 nections of the segments of their sarcode-bodies being shown in such 

 ' internal casts' as are represented in Fig. ^328, B. One of the lowest 

 and simplest forms of this type is that very common one now distin- 



FIG. 328. 



FIG. 329. 



Internal siliceous Casts, representing the forms of 

 the segments of the animals, of A, Textularia, B, Rotalia. 



Tinoporus baculatus. 



guished as Discorbina, of which a characteristic example is represented 

 in Plate xv., fig. 15. The early form of Planorbulina is a rotaline spire, 

 very much resembling that of Discorbina; but this afterwards gives place 

 to a cyclical plan of growth (fig. 17); and in those most developed 

 forms of this type which occur in warmer seas, the earlier chambers are 

 completely overgrown by the latter, which are often piled-up in an 

 irregular ' acervuline ' manner, spreading over the surfaces of shells, or 

 clustering round the stems of zoophytes. In the genus Tinoporus there 

 is a more regular growth of this kind, the chambers being piled succes- 

 sively on the two sides of the original median plane, and those of adja- 

 cent piles communicating with each other obliquely (like those of Textu- 

 laria) by large apertures, whilst they communicate with those directly 

 above and below by the ordinary pores of the shell. The simple or 

 smooth form of this genus presents great diversities of shape, with great 

 constancy, in its internal structure; being sometimes spherical, some- 

 times resembling a minute sugar-loaf, and sometimes being irregularly 

 flattened-out. A peculiar form of this type (Fig. 329), in which the 



