GENUS OPEECULINA: INTERNAL 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 alee, 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 spire, 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 alse are so little extended that the spiral laminse of the successive 

 whorls coalesce not only at the centre, but at some distance around it. After an atten- 

 tive examination and comparison 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 shown in the relative length and breadth of the septal plane, or 

 in the degree in which their alse 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. 



154. 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 spiral 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 spiral 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 Cycloclypeus, 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 



