of the Shell of Operculina Arabica. 167 



confirmed by the circumstance, that the outer margin of the 

 included whorl (fig. 7 c) frequently presents a series of furrows, 

 corresponding to the notches at the inner edge of the septum 

 {h). Each cavity also communicates freely with the interseptal 

 spaces on either side by the smaller apertures and passages last 

 described ; and from this space, as we shall presently see, there 

 was a free passage to the external surface of the shell. 



" The texture of the shell itself differs remarkably from that 

 of any of the MoUusca with which I am acquainted, approach- 

 ing that which I have described in the common Crab (Reports 

 of the British Association, 1847, p. 129). It is everywhere per- 

 forated by a series of tubes of extreme minuteness which pass 

 directly from one surface to another, their openings being plainly 

 visible on each (fig. 16). The diameter of these tubes is about 

 l-7500th of an inch, and their distance from each other about 

 l-15,000th. In a thin vertical section of the shell (fig. 15) 

 they are seen to run parallel to each other, and to be free from 

 sinuosities or interruptions. The whole of this portion of the 

 shell, therefore, is minutely porous. The structure in question 

 can be seldom clearly distinguished in those Nummulites which 

 have had their texture altered by calcareous or siliceous infiltra- 

 tion; but as the appearances which these present correspond 

 closely with those exhibited by specimens of N. Icevigata which 

 have been subjected to the same change, I have no doubt that 

 the tubular structure in question is common to the whole group." 

 " All the Nummulites which I have examined present a remark- 

 able departure from this structure in that portion of the shell 

 which forms the margin of each whorl. Here, instead of an 

 assemblage of minute, closely-set, parallel tubuli, we have a 

 much coarser arrangement, the solid substance being perforated 

 with a smaller number of tubes of two or three times the dia- 

 meter of those last mentioned, which pass in a radiating manner 

 from the inner to the outer surface. Some indications of this 

 difference are seen in fig. 4; but it is much more clearly dis- 

 played at hy by fig. 15, which represents a portion of a very thin 

 section taken in the same direction, and viewed by transmitted 

 light. The openings of these tubes on the outer margin of the 

 whorl are not readily discernible, partly in consequence of the 

 somewhat oblique direction of the orifices, and partly through 

 these being usually covered with a calcareous incrustation. When 

 this has been removed by the application of dilute acid they are 

 easily seen when properly looked for, as was first pointed out to 

 me by Mr. J. Morris." 



Lastly, Dr. Carpenter has observed, in addition to the tubes 

 which run from the punctations on the surface into the chambers 



