REPORT ON THE FORAMINIFERA. 
211 
which they are distinguished. In the comparatively brief descriptions which follow, I 
have made free use of the material ready to my hands in Dr. Carpenter’s Report — indeed 
it would have been difficult to avoid doing so ; and I must refer the reader interested in 
the subject, either as a histological study or in its philosophical bearings, to the fuller 
elucidation he will find in the pages of that work. 
The genus Orbitolites is characterised by a test of discoidab form, and generally of 
comparatively large dimensions, composed of chamberlets arranged either principally or 
entirely in concentric rings, and with an aperture consisting of marginal pores. It 
exhibits a certain amount of variety in the contour and disposition of the earlier chambers, 
which in some species are arranged in a flat spire, and in others assume the annular form 
from the commencement ; but under no circumstances do the spiral segments constitute 
more than a very small proportion of the test, and they are always evolute and non- 
embracing. In this respect the genus differs from Orbiculina, which, though it some- 
times puts on a small number of cyclical chambers, is essentially a spiral type, the earlier 
segments being always so far nautiloid and embracing as to cause a well-marked 
thickening of the central portion of the shell. 
The terms made use of by Dr. Carpenter in his account of the structural features of 
the genus have been for the most part adopted in the following pages, and the subjoined 
woodcut with the accompanying explanation, also borrowed from his Report, will serve 
to illustrate their application. The figure, however, is primarily intended to demonstrate 
the nature of the successive modifications which lead from the simpler to the more 
complex types of structure, and which furnish the basis of distinction between the species. 
Fig. 6. — Diagrammatic representation of the progressive development of the Simple type of Orbitolites into the 
most Complex (after Carpenter). 
p, primordial chamber ; c, c', circumambient chamber ; to, to 1 , m m 3 , to 4 , chambers of successive zones of Orbitolites marginalis, each 
having its single annular canal, ac, and its radial stolon-passage, r ; d, d 1 , rf 2 , chambers of successive zones of Orbitolites duplex, each 
having its single annular canal ac, and its double radial stolon-passage r ; e, el, chambers of intermediate (fossil) form of Orbitolites 
cerrruplanata, each having a pair of annular canals ac, a'c', with an interposed septum i, i, and having its superficial portions, s, s,' 
still in continuity with the median columns to, to ; f, f 1 , p, p, chambers of the typical form of Orbitolites complanata, each having 
its double annular canal, its median columnar portion to, separated from that of the next annulus by the interposed septum i, 
traversed by oblique pores, which appear as marginal pores, mp, at the edge of the disk ; but the superficial chamberlets, s, s, and 
s,sl, alternating in position with the median, and each of {.hem communicating with the annular canals of two zones, as shown at 
ac, aid 
The geographical distribution of the genus extends over a very wide area. The 
typical Orbitolites complanata, in company with the more closely related forms, has its 
