366 M. E. Favre on some Works relating to 



the Aptyclms detached itself from the shell and fell to the 

 bottom of the water, wliilst the shell of the Ammonite was 

 thrown on the shore — or, as M, Zittel has supposed, that these 

 organisms belong to a group of naked Tetrabranchs. 



What is the part played by the Aptychus in the Ammonite ? 

 The Nautilus presenting nothing like it, it was difficult to de- 

 termine its function. Voltz found its analogue in the opercu- 

 lum of the Gasteropoda. Von Buch and Quenstedt regard it 

 as an internal shell. Keferstein has put forward the opinion 

 that the Aptychus might be a protecting organ of the nida- 

 mentary glands of the female Ammonite. M. Zittel has cor- 

 roborated this opinion by several proofs ; and M. Waagen has 

 made it a certainty. 



The normal position of the Aptychus in the Ammonite is so 

 closely related to that of the nidamentary gland in the female 

 Nautilus, that it seems difficult to assign to it a different func- 

 tion. Moreover the soft tissue of this gland has a great re- 

 semblance in its various parts to the structure of the different 

 types of Aptychus, and the form of the Aptychus corresponds 

 very well with that of the outer part of this gland. These vari- 

 ous characters indicate therefore almost certainly the purpose 

 which it serves, although in no living Cephalopod has there been 

 found a similar thickening of the teguments of these glands. 

 We may add, as an indirect proof, that no other organ exists 

 in the Nautilus with the analogue of which the Aptychus 

 could have been connected in the Ammonite. 



It is evident that it could not have served to close the aper- 

 ture of the shell. This opinion, which has been repeatedly 

 maintained, and quite recently by M. Lehon*, has been refuted 

 by M. Waagen. The museum at Munich contains a hun- 

 dred specimens of Ammonites still provided with the Aptychus. 

 Only five of them present the Aptychus placed perpendicularly 

 to the aperture, as M. Lehon has shown it. In all the others 

 it is deeply immersed in the shell in the position here figured 

 (see opposite), a position which corresponds with that of the 

 nidamentary glands of the Nautilus '\. M. Waagen shows 

 besides, by measurements of Amm. steraspis, that the dimen- 

 sions of the Aptychus by no means agree with those of the 

 aperture. Moreover its presence in Ammonites provided with 

 appendages to the aperture proves evidently that it never plays 

 the part of an operculum ; for these appendages often approach 

 each other towards the apex, and would have entirely paralyzed 

 its movements. Keferstein, who had recognized the true 



* Bull, de la Soc. Geol. de France, 1870, vol. xxvii. p. 10. 

 t See, for the position of these glands in the Nautilus, the excellent 

 figure given as the frontispiece to Woodward's Manual. 



