EXPLANATION OF PLATE 32. 55 



Fig. 3. represents in its natural size, a portion of the 

 Siphuncle whiten in Fig. 2. is laid bare along its 

 course through the chambers, d. e. f. In the trans- 

 verse Plate, h, the siphuncular collar is entire, but a 

 Section of another collar in the transverse Plate, i, 

 shows the contraction of the Siphon at its passage 

 through this aperture, and exhibits also the over- 

 lapping, or squamous suture by which the Collar is 

 fitted to the superior and inferior portions of the cal- 

 careous Sheath of the Siphon. See V. I. pp. 247, 

 248. Note. (Original.) 



A similar structure may be seen at the Collars 

 of the transverse Plates of the N. Striatus. See 

 PI. 33. 



across the cavities of the Air chambers. As in the recent Nautilus 

 Pompilius, there is no communication between the interior of the 

 Siphon and that of the Air chambers, so in this fossil shell, there is 

 proof that no communication existed between these cavities. A trans- 

 verse section at a. shows the thin edge of the sheath of the siphuncle, 

 surrounded externally with calcareous spar, and filled internally with 

 Grit. Other Sections of the Siphuncle at b. d. e.f. show the calcare- 

 ous Grit within its cavities to be contracted at its passage through the 

 collars of the transverse plates, and most enlarged midway between 

 one transverse plate and another. 



This fossil affords two proofs that no communication existed between 

 the interior of the Siphuncle and that of the Air chambers. 1st. the 

 calcareous sheath of the Siphuncle is seen at d. e. f. completely en- 

 closing the calcareous grit which forms the cast within it. 2dly, had 

 there been any communication between the interior of the siphuncle, 

 and that of the air chambers, these chambers must have received some 

 portion of the materials of the grit that have filled this Siphuncle: not 

 a particle of grit is found in any one of the adjacent air chambers, but 

 they are all lined, and some of them nearly filled with a crystalline 

 deposite of Carbonate of Lime, disposed in uniform plates around the 

 interior of each chamber, and around the Siphuncle. See Fig. 2. c. 

 c 1 . a. a 1 , a" 3 , a 3 , and Fig. 3. d — k. This deposite can only have been 

 formed from water charged with carbonate of lime, introduced by infil- 

 tration, after the interment of the shell, and filling the chambers which 

 are thus uniformly invested. 



