GENUS OPEECULINA: INTEENAL STEUCTUEE. 



21 



In Plate IV. fig. 5 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 Fora- 

 minifera 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. The like is 

 shown in vertical section in Fig. VII. B. 



152. 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 I. fig. 3). This fissure (Figs. IV., VI. a, a, a) is 

 frequently not to be seen in such a section as is represented in Plate VI. fig. 3, owing to 

 the circumstance that this does not happen to pass through the plane in which it lies ; 

 and it is best brought into view either by making thin transverse sections (as Fig. V.), or 



Pig. V. 



Vertical or transverse section of three outer 

 convolutions of Operculina. 



by breaking a specimen transversely and examining its fractured edges, by which such 

 views will be obtained as are presented in Fig. VIII. (p. 26) A, B, c, D. Besides this prin- 

 cipal aperture, we observe in the septa, especially of the larger chambers, a variable 

 number of secondary pores (Plate I. fig. 3 e), generally circular, and of comparatively 

 small size; these are dispersed without any regularity, as is shown in Figs. V., VI., VIII. 

 They may or may not be brought into view in a horizontal section, according as its plane 

 does or does not happen to pass through them ; but when they are thus traversed (Fig. 

 VI. b, b, 5), it is seen that these secondary pores, like the principal aperture, establish 

 a direct communication between adjacent chambers*. In the irregular formation repre- 



Fig. VI. 



Section of two septa of Operculina through the median plane, showing the secondary pores, I, b, I, in 

 addition to the principal orifices, a, a, at the inner margin of the spire. 



* These secondary pores were first observed by me in Nummulites l<evigata (pp. cit. p. 24) ; I then 

 believed, however, that they do not pass through both layers of the septa, but only establish a communica- 



