328 
EE VIEWS. 
of an inch. In Orbulina two sets of pores appear, the larger, set far 
apart, some l-1500th of an inch in diameter, the spaces between 
these being occupied by perforations which resemble the smaller ones 
of Globigerina. In Operculina and its allies all the pores are equally 
minute and very closely approximated to one another. “ It often 
“ happens, however, that certain parts of the shell are left unehan- 
“ neled by these tubuli; and such are at once distinguished, even 
“ under a low magnifying power, by the readiness with which they 
“ allow transmitted light to pass through them, and by the peculiar 
“ vitreous lustre they exhibit when light is made to fall obliquely 
“ upon their surface. In shells formed upon this type we frequently 
“ find that the surface presents either bands or spots which are thus 
“ distinguished; the non-tubular bands usually marking the posi- 
“ tions of the septa, and being sometimes raised into ridges, though 
“ in other instances they are either level or somewhat depressed, 
“ whilst the non-tubular spots may occur on any part of the surface, 
“ and are most commonly raised into tubercles, which sometimes 
“ attain a size and number sufficient to give a very distinctive charac- 
“ ter to the 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 Flanorbulina 
“ vulgaris. The texture of the shells of this type is much finer than 
“ 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.” And, among the perforate Foraminifera generally 
the extreme differences of diameter which prevail between the pores 
are connected with one another by a most perfect series of grada¬ 
tions. Through these pores, whether large or small, the pseudopodia 
are doubtless protrusible.* 
The arenaceous Foraminifera,f though perhaps an intermediate 
group, come nearer to the perforate than to the non-perforate series. 
Their shell consists of an aggregation of sandy particles closely com¬ 
pacted together by an exudation from the sarcode body. But in 
Valvulina an envelope of this kind completely invests a primary 
vitreous layer, in which perforations have been detected. 
The shell is either monothalamous or polythalamous. All testa¬ 
ceous Foraminifera are monothalamous at first, but in the great 
majority the number of chambers increases by a process of conti¬ 
nuous budding. There are, however, intermediate forms, in which 
the shell, though not divided into chambers, has nevertheless the 
power of indefinite growth. And there are also Polythalamia whose 
shell-chambers have no internal communication with one another. 
Such forms, albeit exceptional, are, from a systematic point of view, 
very significant, and we shall once more have occasion to refer to them. 
* Consult Schultze’s figure of a living Pol/jstomella. 
f See Natural History Review, October, 1861, p. 470. 
