STABILITY OF THE (JKNERA. 55 



cicatrice 1 is the vestige of an opening which placed this ovisack 

 in communication 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 lias lost its ovi- 

 sack. 



In Ammonites and (ioniatites the 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 

 Naut Hides or the Annnonides and (loniatides. 



M. r>arrande 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 

 by M. Barrande to have placed the inollusk contained in the 

 initial chamber in communication with a transitory organ, either 

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

 or to a natatory bladder, etc. 



From the firs! appearance to the final extinction of the Gonia- 

 tid;e and A nnnonit id;e. they always show a typical ovisack; it is 

 therefore impossible to derive them from the Nautilid;e. as sup- 

 posed by the dcvelopmenialist s. This difference has induced M. 

 Munier-Chalmaa (Complex AVm///x. Dec. 2 ( .th. isTtf) to separate 

 the two former from the let rabrancliiate or tentaculiferous cephal- 

 opoda (Nautilidae), and to unite them with the dibranehiate or 

 acetabulifcrous group (Spirulid;e and l>elemnitid;v). which a re 

 provided with an ovisack. 



It still remains to ascertain whether the presence or absence 

 of the ovisack has the systematic importance attributed toil; 

 what is its nature and what is I he 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 embrvogeny of Nautilus shall 

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

 parasitism of the Ponlpe of the Argonaut was agitated with 

 ardor until the day that Dnvernoy showed the embryo of 

 Argonaut to be provided with a shell in the egg. 



