554 
DR. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 
of Peneroplis. But when a portion of the disk is viewed under a higher power 
by transmitted light,—which, through the extreme tenuity of its superficial lamellae, 
brings its internal structure into distinct view (Plate 38, fig. 5),—these lines are seen 
not to be mere surface-markings, as in Peneroplis, but to be the indications of internal 
shelly partitions, which divide each flattened annular chamber into a series of narrow 
cliamberlets, resembling those which I formerly described as constituting the two 
superficial layers of the “complex” type of Orbitolites* Here, however, these 
cliamberlets form but a single plane, as in the “simple” type formerly described; 
and the pores by which the last-formed annulus opens at the margin of the disk are 
arranged in single series (Plate 37, figs. 4, 5). It is worthy of note that these 
pores are not round, like those of ordinary Orbitolites, whether of the “ simple ” or of 
the “ complex ” type ;t but are more or less elongated in the plane of the disk— 
a peculiarity obviously related to its extreme compression. Similar pores are 
seen upon the edge of any zone from ■which the zone external to it has been 
detached by fracture; and it is obvious that they constitute the channels of commu¬ 
nication between the cavitary system of each zone and that of the zones internal and 
external to it; while the marginal series brings the cavitary system of the peripheral 
zone (and, through it, that of every interior zone backwards to the spiroloculine 
“ nucleus ”) into relation with the surrounding medium. 
When a portion of the thin shelly lamella forming either surface of the disk has 
been removed by dilute acid, so as to lay open the cavity beneath (Plate 37, fig. 2), 
it is seen that each zone of cliamberlets lies between two concentric rings of shell, 
a, a, b, b ; and that the radiating partitions, c, while springing from the inner shell¬ 
ring, do not extend to the outer, so that a continuous gallery is there left, into which 
all the cliamberlets open at their peripheral extremities. And when we examine the 
disk by transmitted light (Plate 38, fig. 5), we see it to be from this gallery—not 
from the cliamberlets—that the pores of the shell-ring which incloses it proceed. 
Whilst the structure of the concentric zones forming the peripheral portion of the 
disk thus corresponds in all its essential characters with that of the ordinary 
“ simple ” type described in my former Memoir, the structure of the- central portion of 
the disk is altogether different. The spheroidal “ primordial chamber ” (Plate 38, 
fig. 3, a) is extremely minute, not exceeding 1-1 OOOtli inch in diameter, and from 
this proceeds a compressed shelly tube, which forms a nautiloid spiral around it 
(Plate 38, figs. 3, 5) each successive turn slightly increasing in breadth, so as closely 
to resemble the first-formed part of the spire of Cornuspira. The continuity of its 
cavity, however, is interrupted, usually at about every two-thirds of a turn, by a 
thickening of its wall (Plate 38, fig. 3, b), which seems to have been formed as a sort 
of foreshadowing of a septum at each addition to its length; and thus, as long as the 
growth of the shell proceeds upon the same plan, it is a ‘ spiroloculine ’ Mikola. 
* Phil. Trans., 1856, p. 202, Plate Y., fig. 6, c, c; Plate VII., fig. 12. 
t Ibid., Plate V., figs. 1 and 6, d, d, 
