a new Classification of Ammonites. 367 



function of the Aptychiis, believed that the Anupt jclius served 

 as an operculum : it occurs, however, in the same position as 



A. stcraspis: figure taken from Waagen, PaloDont. xvii. pi. 40. f. 4. 



the former of these organs * ; and it is therefore evident that 

 it fulfilled the same purpose. 



The function of the Aptychus being thus determined, fur- 

 nishes an important point for the determination of the relative 

 arrangement of the organs in the Ammonite, which maj be 

 deduced from that which they occupy in the Nautilus. In 

 this animal the nidamentary gland is situated on the sijjhonal 

 side above the adductor muscle, and outside of the ring of 

 adherence. It is only natural to suppose that the relations of 

 these various organs were the same in the Ammonite. Direct 

 observation serves here to confirm the theory. 



Oppel has remarked in a great number of Ammonites from 

 the limestones of Solenhofen a mark of a peculiar form (Paliiont. 

 Mittheil. pi. 69). M. Waagen has ascertained that this im- 

 pression has precisely the same shape as that of the ring of 

 adherence in the Nautilus (see the foregoing sketch) ; it is the 

 trace of the horny margin of this ring, which has been preserved 

 in consequence of the tranquillity of tlie deposition of the sedi- 

 ments. This trace begins at the margin of the aperture, about 

 in the middle of the sides, follows the spiral of the shell back 

 towards the septum, and then bends forward towards the 

 siphonal side. The Aptychus is above and outside of this 



* See A. planorbis, Sow., in Waagen, Palseontogr. 1869, xvii, pi. 40. 



fi{r. 5. 



