GENUS OPEECULINA: — INTEENAL CHAEACTEES. 
23 
bers is not so extended, those of each whorl being bounded internally by the external 
margin of the preceding. Passing from this, however, to forms which are slightly 
elevated in the centre (Fig. VII. e), we find not only that the breadth of the septal plane 
is increased, but that the cavity of each chamber is extended into two alm^ which are 
more or less prolonged over the enclosed whorls ; so that the spiral lamina of the invest- 
ing whorl is kept by their interposition from coalescing with that which it embraces, 
except in the central region. The degree of this extension, however, varies in different 
convolutions ; the alee being usually smaller in the chambers of each consecutive whorl, 
and being absent altogether from those of the last, — a transition well shown in Fig. V., 
which also exhibits the marked variation in the general proportions of the chambers 
that may present itself between the inner and the outer whorls. In forms which are 
distinguished by a yet more turgid sphe, the proportions of the chambers are very con- 
siderably modified ; but among these there is a very considerable difference in the degree 
in which the chambers of the later whorls are prolonged over the earlier; thus in 
Fig. VII. A, whose centre is on a level with the rest of the spire, we see the alse pro- 
longed so as quite to reach that region ; whilst in d and f, which have the central region 
depressed, though the general proportions of the chambers correspond with those of 
the preceding, the alae are so little extended that the spiral laminae of the successive 
whorls coalesce not only at the centre, but at some distance around it. After an atten- 
tive examination and compaiison of a great number of vertical sections, I feel justified 
in affirming that no importance can be attached either to the form and proportions of 
the chambers, whether shoum in the relative length and breadth of the septal plane, or 
in the degree in which then.' alae are prolonged over the enclosed whorls, as furnishing- 
characters of specific difference ; seeing that it is not only found to vary gradationally 
when a sufficiently large series of specimens is compared, but that it is often equally 
inconstant in different parts of the same individual. 
164. The spiral lamina, in typical specimens, is much thicker in the inner than in 
the outer convolutions, that of the last whorl being always comparatively thin (Fig. V.); 
and a careful examination of transparent vertical sections makes it apparent, that the 
greater thickness of the spiral lamina of the earlier whorls is partly due to the prolonga- 
tion of that of the later whorls over them, and to its coalescence with them. I have not 
been able to satisfy myself that the sphal lamina of the last whorl is thus extended over 
the preceding whorls ; and it has rather appeared to me to be merely applied to the 
margin of that which it surrounds. The difficulty of certainly determining this point, 
chiefly arises from the circumstance that the sphal lamina of the last whorl is reduced 
' in thickness in proportion to its extension, as if in consequence of a limitation of the 
amount of calcareous matter employed in its formation. The spiral lamina is made up 
of a variable number of lamellae of a minutely tubular substance (Plate IV. fig. 9), so 
exactly corresponding with that which I have already described in Nummulites (loc. cit.) 
and in Cyclochjpens, that any detailed description of it is unnecessary. I have to add, 
however, that in examining extremely thin transverse sections of this tubular shell-sub- 
stance with a sufficiently high magnifying power, I have been able to perceive that the 
