parti] ON THE MO"RPnOLOGT OF THE AMMOXTTE SEPTUM. 35 



VI. The Formattox of the SEPTTj]\r. 



Evidence bearing upon the mode of deposition of the septum is 

 scanty. In an adult specimen of Daciijlioceras commune from 

 Grantham, the last septum Avas found to be only in part secreted. 

 Its suture was clearly visible as a white line at the umbilical angles. 

 Here the secretion of the sej^tum was evidently well-advanced, but 

 elsewhere it could be seen onl}^ as a faint dark line — possibly due to 

 a conchiolin membrane. Septal sections seemed to indicate that 

 the septum was incomplete internalh'; but a definite statement 

 cannot be made, for it had evidently been disrupted when the shell 

 was becoming filled with mud. The evidence shows that the 



Fig. o. — Last tivo suture-lines of two ammonites showing the 

 last suture incompletely formed. 



'^iVv 



■^fV'v^^Vlpv 



Sb = Dactylioceras commune, Grantham. 

 h = Polymorph ites Jupiter, Old Dalby. 



Diameter = 57 mm. 

 Diameter = 18 mm, 



secretion commenced in the region farthest removed from the 

 siphuncle. Partly formed septa, showing the same peculiarities, 

 have also been noticed in Fol// morphites p(i)iter d'Orhigny (fig. 6h). 



The formation of the septum in ammonites seems to have taken 

 place in the same manner as in living Nautilus. It has long 

 been known i that in the latter the deposition of calcareous 

 mattei- begins at the sides of the shell, and proceeds towards the 

 siphuncle. Willey (19U2) observed further that the secretion of a 

 septum was preceded by the formation of a conchiolin membrane, 

 upon which the calcareous matter was deposited. 



The septa, just described for Dacti/lioceras and Foh/morphites, 



^ F. A. Bather, ' Growth of Cephalopod Shells ' Geol. Mag. dec. 3, vol. iv 

 1887) p. 447. 



j)2 



