GENUS OPERCULINA. 



255 



in different parts of the same individual. Commencing with one of the flattest variety, Fig. 

 XLII, c, we observe that the breadth of the septal plane is very small in comparison with its 

 length ; and that although the spiral lamina of each whorl except the last obviously extends 

 itself over the preceding whorls, the cavity of the chambers is not so extended, those of each 



Fig. XLII. 



^ -*JjiiJl!liLL_ 



Vertical or transverse section of six specimens of Opeicidina, exliibiting marked variations iu the form and proportions 



of the convolutions. 



whorl being bounded internally by the external margin of the preceding. Passing from this, 

 however, to forms {c) which are slightly elevated in the centre, we find not only that the 

 breadth of the septal plane is increased, but that the cavity of each chamber is extended into 

 two «/^, which are more or less prolonged over the enclosed whorls; so that the spiral 

 lamina of the investing 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 alae 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. 

 XL, 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 considerably modified ; 

 but among these there is a considerable difference in the degree in which the chambers 

 of the later whorls arc prolonged over the earlier; thus in Fig. XLII, a, whose centre is on a 

 level with the rest of the spire, and in e, whose centre is elevated, we see the ate prolonged 

 so as quite to reach that region ; whilst in d and f, which have the central region depressed 



