270 NAUTILUS SYPHQ, ETC.. 



SECTION V. 



NAUTILUS SYPHO, AND NAUTILUS ZIC ZAC. 



The name of Nautilus Sypho* has been applied to a ver]. 

 curious and beautiful chambered shell found in the Tertiary 

 strata at Dax, near Bourdeaux; and that of Nautilus Zic 

 Zac to a cognate shell from the London clay. (See P1..43» 

 Figs. 1, 2, 3, 4.) 



These fossil shells present certain deviations from the 

 trdinary characters of the genus Nautilus, whereby they 

 in some degree partake of the structure of an Ammonite. 



These deviations involve a series of compensations and 

 pecuhar contrivances, in order to render the shell efficient 

 in its double office of acting as a float, and also as a defence 

 and chamber of residence to the animal by which it was 

 constructed. 



Some details of these contrivances, relating to the Nauti- 

 lus Sypho will be found in the subjoined note.f 



* This shell has been variously described by the names of Ammonites 

 Atun, Nautilus Sypho, and N. Zonarius. (See M. de Basterot. Mem. Geol- 

 de Bourdeaux.) 



t The transverse plates, (PI. 43, Fig. 1, a. a'. a2«,) present a peculiarity of 

 structure in llie prolongation of the collar, or siphuncular aperture entirely 

 across the area of the air-chambers, so that tiie whole series of transverse 

 plates are connected in one continuous spiral chain. This union is effected 

 by the enlargement and elongation of the collar for the passage of the 

 siphuncle into the form of a long and broad funnel, the point of which b. fits 

 closely into the neck of the funnel next beneath it, g. whilst its inner margin, 

 resting upon the arch of the subjacent whorl of the shell, transfers to this 

 iLTch a portion of the external pressure upon the transverse plates, thereby 

 adding to their strength. 



As this structure renders it impossible for the flexible siphuncle to 

 expand itself into the area of the air-chambers, as in other Nautili and 

 in Aintnonites, the diameter of each funnel is made large enough to allow 



