REPORT ON THE FORAMINIFERA. 
331 
The dimensions which have been given by no means represent the entire range in 
size, for the specimens originally described by d’Orbigny, from West Indian shore-sands, 
were only one-tenth of a millimetre in diameter. No doubt individuals as minute as this 
are occasionally found, but about 0‘5 mm. is the average measurement of recent specimens, 
as found in very deep or very shallow water, and such shells generally consist of five or 
sis convolutions of nearly even width. The exceptionally large specimens frequently 
met with in localities rich in arenaceous Foraminifera, at depths of from 300 to 1000 
fathoms or thereabouts, attain a diameter of nearly 3 mm., and often present 
from fifteen to twenty convolutions. In these the early whorls are narrow, and there is a 
slow gradual increase in width, which is rather more rapid in the later stages. The 
successive convolutions are sometimes slightly embracing, and as the peripheral edge of 
the test is always rounded, the shape of the tube in transverse section depends a good 
deal upon the extent to which its sides overlap, and may be oval, circular, semicircular, 
or crescentiform. 
Turning for a moment from the consideration of the characters of the recent test, 
the following note from the Monograph of Carboniferous and Permian Foraminifera, 
p. 73, apropos of the earliest fossil representatives of the species, may not be out of 
place : — “ Wherever it [Ammodiscus inccrtus ) exists as a palaeozoic fossil it appears in 
large numbers, and the specimens present a correspondingly wide range of variation in 
minor characters. Many specimens are just such as might be dredged at the p resent 
day on our own shores, consisting of five or six convolutions in one plane of a non-septate 
tube, the convolutions nearly uniform in breadth, and the tube having an approximately 
circular transverse section. The examination of a large number of individuals reveals 
many little modifications of these simple typical characters. Sometimes the number of 
convolutions is smaller and their width greater, forming a test of similar diameter and 
without increase of thickness, and in such the tube presents a long oval instead of a 
circular transverse section. Other examples show a tendency in the successive convolu- 
tions each to embrace, to a limited degree, that immediately within it, and the section 
of the tube is then more or less crescentiform. In some of the larger complanate shells 
the spiral tube increases in width with each succeeding circlet. Lastly, it is not at all 
uncommon to find the shell- wall thickened, especially near the centre of the disc, the 
excavated sutural line filled up, and the test assuming thereby a more or less lenticular 
or biconvex figure. In these instances the calcareous cement is largely in excess of the 
arenaceous material, the surface of the test is nearly smooth, and even permits, by a sort 
of transparency, the course of the spiral cavity in the interior to be traced. These modi- 
fications, in addition to many irregularities in external contour, arise from what may be 
regarded as accidental circumstances, and present no ground for specific or varietal 
subdivision.” 
From this description it will be gathered that the Ammodiscus inccrtus of the Car- 
