FORAMINIFERA FROM THE NORTH ATLANTIC AND ARCTIC OCEANS. 399 
Operculina ammonoides is very common in the mixed sands from Norway (Mac Andrew 
and Barrett). On the Irish plateau of the North Atlantic it is common at 43, 78, 90, 
223, and 415 fathoms ; and rare at 200 fathoms. It abounds in the North British seas ; 
in Professor Williamson’s Monogr. it appears under the name of Nonionina elegans. It 
is found also in the Mediterranean and Red seas, and at Australia and Fiji. 
Genus Polystomella. 
Polystomella crispa, Linn., sp. Plate XIV. fig. 24 (Arctic) ; Plate XVII. fig. 61 a, 61 b 
(North Atlantic). 
Polystomella comprises many closely allied forms, which, on account of their appa- 
rent dissimilarity, have been usually grouped under Nonionina and Polystomella. Their 
differences, however, are not sufficient to destroy the value of their correspondences in 
structure. The shells are symmetrically discoidal, either lenticular or subglobular, 
more or less Nautiloid, having from about fifteen to thirty, or many more, neatly fitting, 
more or less sickle-shaped chambers, with the aperture at the base of the septa; and 
this may be either a simple low arch-like opening, or it may be crossed by bars so as to 
be a grating, or a row of pores ; this multiplicity of stolon-passages is the condition 
which gave the name to this genus in particular, and to the “Foraminifera” altogether*. 
The gradations from the simply notched septum of some Nonionince , to the barred aper- 
tures of others (N. Faba, Fichtel and Moll, sp.), and thence to the curved row of pores 
in Polystomella proper, are very well marked in numerous modified varieties. Another 
feature of the genus is the masking of the septal furrows of the shell, by “ retral pro- 
cesses,” or lobes on the posterior edges of the chambers, connected by bridges of exoge- 
nous shell-matter to the fronts of the preceding chambers, and thus forming pits or 
“ fossettes” along the septal lines. The mouths of the canal-system open into the “ fos- 
settes;” but the latter are not a part of that system. The processes and the bridges or 
bands vary much in thickness, in proportion to the higher standing of the more strongly 
grown varieties of this species ; and this increase of shell-matter on the surface of the 
shell, until it has a sculptured or basket-work appearance, accompanied more or less with 
keel, spines, and umbones, is also traceable through very gentle gradations. 
The “ bridges ” occur freely, in P. Arctica and other forms, when the retral lobes are 
nearly obsolete, and thus they form crenulations on the edges of the chambers. 
As the soft parts of the animal afford us no distinctive specific characters, all these 
modifications of shell-structure fall into a series of varietal differences among the indi- 
viduals of one species, subject to different conditions of existence and consequent modes 
of growth. 
In its symmetry of shell Polystomella resembles Nummulina, but it has a canal- 
system different from that of the latter ; and, though the aperture in Nummulina is in 
the same position (at the base of the septum) as in Nonionine Polystomellce, yet the very 
* As being distinct in so much from the single-tubed Cephalopods, with which they were classed. 
3 h 2 
