GENUS OPERCULINA. 



253 



plane that shall traverse all the whorls to the central cell; by choosing a sufficiently flat 

 specimen, however, a sufficiently near approach to this may be made to i)ring into view the 

 general disposition of the chambers. This is much less regular than would be supposed from 

 a superficial examination of the exterior. For while there is a certain general average in the 

 proportion which their long diameter (that is, the breadth of the whorl) bears to the short (or 

 the distance between the septa) which may be slated as about 4^ to 1, this is by no means 

 constantly maintained, the long diameter being sometimes as much as 7 times, and sometimes 

 no more than 2| times, the short. The ordinary course of the septa, too, is often strangely 

 departed from. There are few individuals which do not present — besides abnormal sinuosities, 

 greater or less in degree — very marked irregularities in the conformation of the chambers, 

 analogous to those which occur in Numwulifeii. Frequently a septum, instead of passing con- 

 tinuously from the inner to the outer margin of the whorl, stops short witliout reaching the 

 latter, and bends backwards to join the last-formed septum ; and sometimes a second septum 

 unites itself to the first in the same manner. The abortion is often still more marked ; thus 

 in Fig. XXXIX we see three septa thus interrupted, of which two do not traverse half the 

 distance, and the third not a quarter, thus dividing the space between two complete septa into 

 three small chambers along the inner margin, and one large irregular chamber extending to 

 the outer. 



Fig. XXXIX. 



Irregular disposition of septa in Operculinct: — «, a, n, uormal apertures at inner margin of spue; h, b', b", apertures 

 of communication between abuormallj divided clumbers. 



Generally speaking, the more nearly we approach the centre of the spire, the more regularly do 

 we find the septa disposed, until we come into close proximity with the central chamber. In 

 Plate XVII, fig. 6, we have an illustration of what may be considered the normal mode of 

 commencement of the spire ; from which it will be seen that it originates, as in Foraminifera 

 generally, in a spheroidal cell, from which others are successively developed around it, the 

 earlier chambers having no very definite shape, but those which succeed them gradually 

 coming to assume the characteristic form and proportions. 



440. Each chamber communicates with the neighbouring chamber on either side, by a 

 long narrow crescentic fissure left by the non-adhesion of the septum to the outer margin of 

 the preceding whorl (Plate XVII, figs. 1, 2, e, e, e). This fissure is best brought into view 

 either by making thin transverse sections (as Fig. XL), or by breaking a specimen transversely 

 and examining its fractured edges, by which such views will be obtained as are presented in 



