THE MABQUIS DE FOLIN ON THE O^CIDiE. 255 



(upon which it will soon have to support itself ), it is indispens- 

 able to regard the animal as already established beyond the chosen 

 point, in order there to trace out the plane of obliteration (A B, 

 fig. 1). From that moment the portion A B C D will remain 

 empty and useless up to the time of decollation. The first layer 

 will be deposited at the points a&h, and the process of formation 

 will be continued in the planes c d, c' d', c" d", &c., all parallel to 

 the plane A B. But as this is accompanied with a continual 

 narrowing of the space to be filled up, eventually the wall 

 ace' c" s d" d' d b is formed, which terminates at the summit or 

 apex s; and this constitutes the outer layer of the septum. 

 "With respect to the thickening of that part, we may consider it 

 certain that it is accomplished after the closure has been com- 

 pleted. 



It should be observed that in the cases in which the septum as- 

 sumes the form a s b, fig. 2, and which it presents in several re- 

 markable species, the process must be efiected from s to e, and from 

 e to e' before it is finished at cfd, the portion asb being always 

 occupied, and the point of insertion of the muscley being always 

 placed pretty near the " plane of obliteration." We may also re- 

 mark that the thickness always increases as the septum approaches 

 the axis, and that it is greatest on the imaginary line proceeding, 

 from the apex from s to s', fig. 1. But this can only be effected 

 by the superposition of concentric layers of constantly diminishing 

 diameters, and whose borders a,h,c,d,e,f, fig. 3, are further and 

 further remote from the walls of the tube, A B C D. 



From the preceding remarks it would follow : (1), that whatever 

 may be the character of the septum, and consequently its external 

 form, the interior part does not correspond with this form ; (2), that 

 the septum always constitutes a firm dissepiment, upon which a 

 site for the muscular impression is reserved. Nevertheless, in 

 the species furnished with mammillated or hemispherical septa, the 

 internal surface is slightly concave. 



Mr. Phillip Carpenter, in his Monograph of the Csecidse, ex- 

 presses the opinion that the septum is perhaps an integral part 

 of the shell, which advances from time to time by additional layers. 

 That eminent author bases his hypothesis on the following fact. 

 A specimen of Brochina glabra seemed to be deprived of a septum ; 

 having been broken accidentally, the cicatrix was visible at the 

 end of one of the fragments. Such a position seemed abnormal, 

 and appeared to justify the supposition above cited. Neverthe- 



