610 
THE VOYAGE OF H.M.S. CHALLENGER. 
these circumstances the conclusion seems obvious that the shell of Orbulina. in its typical 
condition has no general aperture, in the sense in which that term is ordinarily employed. 1 
With reference to the existence of a distinct series of larger pores it is interesting to 
note the collateral fact, that in certain other genera of Foraminifera, where a final 
segment of abnormal size is formed, such, for example, as the balloon-shaped chamber of 
Cymbalopora bulloides, or the inflated terminal segment of Pulvinulina lateralis , the 
ordinary aperture is more or less replaced by a number of exceptionally large perforations 
scattered over the face of the outermost chamber. 
Thick-shelled bottom specimens of Orbulina frequently exhibit a curious lamination of 
the walls of the test, affording clear evidence that the increase in thickness has taken place 
not as a continuous deposit but by the formation of successive layers of calcareous matter. 
An illustration of this phenomenon is shown in PI. LXXXI. fig. 26, which represents a 
shell formed of at least four concentric laminae. This structure is best seen in old and 
worn specimens, and it is quite possible, though the layers are well-defined and the 
enclosed spheres loose and easily separated, that the entire wall may have been compact 
and to all appearance homogeneous during the life of the animal ; in other words, that 
the visible separation into laminae may be the first stage in the disintegration of the shell. 
There are, however, rare instances of living surface-specimens with a double shell-wall, 
and in one case a threefold shell has been observed ; but in all these the calcareous 
laminae are of extreme tenuity, and bear little actual resemblance to the coats 
of the bottom-specimens. 
As has been already stated (p. 607), bottom-specimens have occasionally been 
found, which, like those from surface gatherings, contain a polythalamous nucleus. 
These, however, are comparatively rare and of somewhat local distribution. In dredged 
specimens, as a rule, the cavity of the test is undivided and seldom contains any trace 
of an internal skeleton ; if, therefore, the Globigerina- like inner shell is an invariable 
feature of the early stages of growth, which is by no means proved, it is probably 
absorbed during the process of thickening of the outer walls. 
In bottom-ooze, in which Orbulina abound, two-chambered shells are not unfrequently 
met with. They consist of the normal spherical test with an adherent supplementary 
chamber. Sometimes the added segment is larger than the original test, as in two 
of the figured specimens, PL LXXXI. fig. 20, and PL LXXXII. fig. 2 ; in other cases 
it is of about the same size, as shown in Pl. LXXXI. fig. 21. Such forms constitute 
the Globigerina bilobata of d’Orbigny. More rarely shells with two supplementary 
chambers present themselves, of which Pl. LXXXII. fig. 3 is an example. In all these 
cases the chambers have the true Orbuline character, and the test has nothing resembling 
a Globigerine aperture, indeed it seldom possesses any aperture at all. From a morpho- 
1 Shacko’s recent observations entirely confirm this view (op. cit.), p. 433. On the other hand it is right to state 
that Wallich entertains the contrary opinion (Deep-sea Researches on the Biology of Globigerina, p. 74). 
