GENUS OPEECULINA: — EXTERNAL CHARACTERS. 
17 
to increase upon such a plan ; for the compression of the spire always shows itself in the 
third whorl, if it has not previously done so, and is accompanied with a corresponding 
augmentation of its breadth, so that the septal plane becomes narrowed and elongated, 
as is seen in comparing tig. 4 with tig. 3. That no basis for specific distinction is 
afforded by the most marked differences in the form of the spire, as shown in the pro- 
portions of the septal plane (a, b, c, d), will be made evident by the examination of such 
a transverse section as is shown in Fig. V. (p. 21) ; in which it will be observed that 
differences equal to those presented by the most compressed and the most turgid forms 
of the spire, are exhibited by the successive convolutions of one and the same individual. 
146. Another marked feature of difference among the specimens of this collection, is 
the depression or elevation of the central region relatively to the peripheral. In what 
I have assumed as the typical form, the central region presents the same general level 
with the rest, though the umbilicus itself is often marked by a prominent tubercle. In 
such forms as are represented in Plate III. figs. 1 and 2, the umbilical region is rather 
depressed than elevated ; and this depression is often observed in older specimens whose 
early growth has taken place on this type. But there is a group of specimens, of which 
three successive ages are represented in Plate III. figs. 3, 4, 5, 8, that have the whole 
central region so exceedingly prominent as to form a cone, whose apex is marked either 
by one large tubercle, or by a cluster of smaller ones ; and this conformation gives so 
peculiar a physiognomy to the shells which present it, that few systematists would hesi- 
tate in placing them apart as specifically different from the rest. On a careful compari- 
son of a large number of indmduals, however, it becomes apparent that this difference, 
like the preceding, is gradational ; every degree of prominence being traceable from the 
individuals which have the umbilicus marked only by a tubercle, as in fig. 7, through 
those in which the region generally is slightly elevated, as in fig. 8, to those in which it 
presents the most marked projection, as in figs. 3-5. We shall presently find 163) 
that this difference depends mainly on the degree in which the investing layer, prolonged 
from the later convolutions over the surface of the earlier, is separated from that surface 
by the extension of the alar prolongations of the chambers of the investing whorls ; as 
to which point there is a most remarkable diversity, not only among different individuals, 
but between the several convolutions of the same individual. 
146. A fourth very obvious character of differentiation among the individuals of this 
collection, consists in the presence or absence of tubercles on the septal bands. In what 
I have described as the typical form (Plate III. fig. 7), there are no considerable pro- 
minences over the greater part of the surface ; the septal bands are generally smooth 
and continuous ; and it is only in the central region that we observe any departure from 
this uniformity, the umbilicus being occupied by a small tubercle, and the smooth septal 
bands being replaced in the first whorl by moniliform rows of little tubercles. In other 
instances, however, we find not only the central tubercle, but the rows of tubercles 
marking the septa, much larger and more prominent ; and this marking-out of the septal 
bands by elevated tubercles is not limited to the first whorl, but extends to the second, 
and even to the third, as is well shown in fig. 6. The specimens whose central region is 
MDCCCLIX. 
D 
