of the Shell of Operculina Arabica. 173 



the external surface of the shelly and the laminated structure, 

 which it presents on a vertical section, thus formed ; while the 

 addition of chambers would appear to commence from the open- 

 ing in the septum close to the preceding whorl, and an inter- 

 septal vessel, arising as before described from its marginal plexus, 

 to extend outwards, on either side, pari passu with the chambers 

 to the circumference, which it may fall short of or not, as already 

 stated. Again, it would appear that this addition does not take 

 place singly, but that there are always two or more chambers 

 (fig. 8 b, by b) in process of formation, the last being the smallest, 

 and that, one after another, they gradually reach the margin. 

 I have come to this opinion, not from the recent specimens of 

 Operculina in my possession, in which, as before stated, all the 

 last-formed chambers are broken, but from having observed the 

 ochraceous casts of microscopic nautiloid species of Foraminifera 

 which have been fossilized, to present this form, when dissolved 

 out from their matrix. 



Analogy to Porifera. — When Dujardin, guided by the sug- 

 gestion of Bory St. Vincent, was struck with the analogy which 

 exists between the filamentous prolongations of Foraminifera and 

 those of the Amoeba or Proteus, he could have little thought, 

 that however nearly the latter might be allied to the Sponges, 

 the former would be found so similar to them in their compact 

 structures. Who, indeed, looking at the nautiloid form of a 

 foraminiferous shell and an amorphous piece of sponge, would 

 say, that they bore the least resemblance to each other ? Yet 

 they are, as we have seen, most intimately allied, both in their 

 fleshy and their compact structures. It must be now generally 

 allowed, that the rhizopodous nature of Foraminifera is identical 

 with that of the Amoeba or Proteus, and through the latter with 

 the sponge-cell ; and in addition to this, we have the former, at 

 least the genus Operculina, still more nearly allying Foraminifera 

 to the Sponges, by possessing a spicular structure, if not a cir- 

 culating system also, like that of sponges. It is curious too, that 

 without any reference to the use of the pores in these two orders 

 of animals, they should have received names of the same signi- 

 fication, as if the intimate relationship which is now found to 

 exist between them was instinctively anticipated, before it was 

 proved by demonstration. The genus of Porifera to which 

 Operculina comes nearest is, of course, the calci-sponges, that 

 called Grantia, after their distinguished discoverer Dr. Grant ; 

 and of this genus, it would seem to approach nearest to the 

 tubular species, which have but one vent. 



Structure of the Shell of Operculina compared with Nummulites. 

 — It will be very gratifying to those whose investigations of the 

 structure of Nummulites must have been attended with so much 



