254 



FAMILY NUMMULINIDA. 



Fig. XLIII (p. 258), A, B, c, d. Besides this principal aperture, wc observe in the septa, espe- 

 cially of the larger chambers^ a variable number of " orbuline " or secondary pores (Plate XVII, 



Fig. XL. 



Vertical or transverse section of three outer 

 convolutions of OperatUiia. 



figs. 1, 3,/;/), generally circular, and of comparatively small size; these are disposed without 

 any regularity, as is shown in Figs. XL, XLIII. They may or may not be brouglit 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. XLI, b, h, b), it is seen that these secondary pores, like the 

 principal aperture, a, «, establish a direct communication between adjacent chambers. In the 



Fig. XLI. 



Section of tke two septa of Operculina through the median plane, showing the secondary pores, h, h, h, in addition to 

 the principal apertures, o, a, at the inner margin of tlie spire. 



irregular formation represented in Fig. XXXIX, these secondary pores {h, I', b") present a 

 disposition which would seem to indicate that they are scarcely less important for the mainte- 

 nance of the communication between the chambers, than is the principal aperture itself; and 

 it is curious to observe how exactly they repeat at h' and b" that mode of communication 

 between two segments on the inner side and one on the outer, which is so characteristic of 

 Or/j'doUtes and Orbiculinu. These secondary pores may be considered as representing, in the 

 more elevated type with which we are at present concerned, the multiple pores which consti- 

 tute the sole communication between the chambers in PeneropUs (1 124). Every septum is 

 composed of two laminae (Plate XVII, fig. 1, d, d), each being the proper wall of one of the 

 chambers which it separates; these laminae are commonly in close apposition with eacli 

 other; but they separate at certain spots, to give passage to the very curious system of 

 interseptal canals to be presently described. 



441. A comparison of numerous vertical sections brings into view a most remarkable 

 variety in the form and disposition of the chambers, not merely in different individuals, but 



