STABILITY OF THE GENERA. 55 



cicatrice is the vestige of an opening which placed this ovisack 

 in coininiinicatioii with the initial air-chamber of the shell; but 

 he has never seen this supposed ovisack. which is hypothetical. 

 For him. the Nautilus is a cephalopod which has lost its ovi- 

 sack. 



In Ammonites and (Joniatites I he initial disposition is entirely 

 different. The ovisack is plainly visible, globular or ellipsoidal, 

 more dilated than the part contiguous to the chambered spire. 

 No appearance of a cicatrice. It suffices, consequently, to 

 examine the first chamber of a cephalopod to class it among the 

 Nautilides or the Ammonides and (ionia tides. 



M. Barrande has shown that the initial appearance of the shell 

 of Nautilus is exhibited without any change through all the 

 geological periods to the present time. The fissure is supposed 

 bv M. Barrande to have placed the mollusk contained in the 

 initial chamber in communication with a transitory organ, either 

 a vitelline vesicle (which, to M. Fischer, appears inadmissible) 

 or to a natatory bladder, etc. 



From the first appearance to the final extinction of the Gonia- 

 lidje and Ammonitida\ they always show a typical ovisack ; it is 

 therefore impossible to derive them from the Nautilidie, as sup- 

 posed by the developmentalists. This difference has induced M. 

 Munier-Chalmas (Comptes Itendus, Dec. 29th, 1873) to separate 

 the two former from the tetra branchiate or tentaculiferous cephal- 

 opoda (Nautilidiv). and to unite them with the dibranchiate or 

 acetabuliferous group (Spirulida? and Belemnitidae), which are 

 provided with an ovisack. 



It still remains to ascertain whether the presence or absence 

 of the ovisack has the systematic importance attributed to it ; 

 what is its nature and what is the purpose of the cicatrice. The 

 word ovisack may be badly chosen because it supposes the 

 existence 'of calcareous envelopes to the eggs. 



One might discuss and wonder a long time on this subject 

 .until a direct observation on the embryogeny of Nautilus shall 

 give us the true solution. It is thus that the question of the 

 parasitism of the Poulpe of the Argonaut was agitated with 

 ardor until the day that Duvernoy showed the embryo of 

 Argonaut to be provided with a shell in the egg. 



