20 DE. CABPENTEE'S EESEAECHES ON THE FOEAMINIFEEA. 



disconnected from one another. In Plate I. fig. 3, an attempt is made to bring together 

 in an ideal representation the most important features of the internal structure of Oper- 

 culina, as disclosed by these two methods of examination ; the warranty for all its details 

 being supplied by the delineations of the separate parts that are given in other figures, 

 which are all taken from preparations in my possession. 



151. Owing to the circumstance that large specimens of Operculina are rarely if ever 

 quite flat, it is next to impossible to make a section through the median plane that shall 

 traverse all the whorls to the central cell ; a sufficiently near approach to this, however, 

 has been made in the section represented in Plate VI. fig. 3, and Plate IV. fig. 10, to 

 bring 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 distance between the septa), which may be stated as 

 about 4^ to 1, this is by no means constantly maintained, the long diameter being some- 

 times as much as 7 times, and sometimes no more than 2^ times, the short. The ordi- 

 nary course of the septa, too, is often strangely departed from, as is seen in Plate VI. 

 fig. 3. There are few individuals which do not present besides abnormal sinuosities, 

 greater or less in degree very marked irregularities in the conformation of the cham- 

 bers, analogous to those which I have described inNummulites*. Frequently a septum, 

 instead of passing continuously from the inner to the outer margin of the whorl, stops 

 short without 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, as shown 

 at a, Plate VI. fig. 3. The abortion is often still more marked ; thus at 6 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 four small cham- 

 bers along the inner margin, and one large irregular chamber extending to the outer. 

 Another case of the same kind is seen in an earlier part of the same whorl; and in 

 Fig. IV. an additional illustration on a larger scale is given of the like irregularity, for 



Fig. IV. 



Irregular disposition of septa in Operculina : a, a, a, normal apertures at inner margin of spire ; 

 b, V, b", apertures of communication between abnormally divided chambers. . 



the purpose of showing the arrangement of the communicating passages. 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 cell. 



* Quarterly Journal of the Geological Society, vol. vi. 1850, p. 23. 



