REPORT ON THE FORAMINIFERA. 
215 
tier. The adult shell seldom measures more than |-th inch (5 mm.) in diameter, with a 
thickness of about y^th inch (0 '1 5 mm.), and frequently does not attain more than half 
these dimensions. 
The centre of the test is occupied by a “ nucleus,” consisting of a rounded primordial 
cell opening into a second or circumambient chamber, and this is followed by a number 
of arched segments arranged spirally in one plane. The spiral segments increase 
progressively in length and in degree of curvature, until by the meeting of the 
two ends the normal cyclical plan of growth is established, after which the remainder 
of the test is constructed of concentric zones. Except just at the commencement, 
the whole of the segments are subdivided into chamberlets by radial partitions. 
The septal walls and secondary septa are much thicker than those of Orbit olites 
tenuissima, and the chamberlets, as seen on the surfaces of the test, are nearly round in 
outline. 
The connection between the chamberlets of the same chamber is maintained by lateral 
stoloniferous tubes or galleries just within the peripheral wall. The communication 
between the successive annuli is established by means of radial passages proceeding from 
the lateral galleries, and not from the chamberlets themselves, except when additional 
chamberlets are interposed to provide for the increasing length of the segments. The 
more or less regular alternation of the chamberlets of the successive zones is due to this 
arrangement. The radial passages of the final chamber form the aperture of the test, and 
appear externally as a line of equidistant pores on the peripheral face. 
The simple structure of the test, its spiral commencement, and the rounded form of 
the chamberlets, are sufficient, taken collectively, to distinguish Orbitolites marginalis 
from any of its allies ; and the species is more likely to be confounded with the com- 
planate varieties of Orbiculina (PI. XIY. fig. 9) than with the forms generically 
related to it. In Orbiculina, however, the early chambers are not only spiral but 
nautiloid or embracing, and the test in consequence exhibits a well-marked thickening of 
the umbilical region, which is never observed in Orbitolites. 
In its geographical distribution, Orbitolites marginalis is closely associated with the 
complex type Orbitolites complanata, the home of both being the shallow-water sands of 
tropical and sub-tropical seas ; but it is not so abundant as the latter species, nor quite so 
widely diffused. It has been noticed at thirteen Challenger Stations, of which eleven 
are in the neighbourhood of the islands of the Pacific, the depths ranging from six 
fathoms to 620 fathoms ; the remaining two are, — off - Culebra Island, West Indies, 
390 fathoms, and off Bermuda, 435 fathoms. It has been found also in shallow 
dredgings from the Mediterranean and from the Eed Sea, and in shore-sands from 
Madagascar. 
There is no satisfactory record of its occurrence in the fossil condition. 
