222 DISTRIBUTION OF THE MOLLUSCA IN TIME. 



agree in considering as purely specific. Even tlie Triassic 

 Nautilides show less affinity to existing species than do the 

 primitive forms. The theoretical evolution of the cephalopods, 

 like that of the Trilobites, appears to us to be imaginary, 

 without any foundation in fact.* 



Dr. Paul Fischer, in a notice of Barrande's work, whilst 

 acknowledging the strength of the facts and observations brought 

 forward by that distinguished palaeontologist against the 

 development theorj^, does not consider them conckisive : The 

 type Groniatites, says M. Fischer, has always been considered by 

 evolutionists as a natural transition between the Nautilus with 

 its very simple partitions and the foliaceous sutures of the 

 Ammonite ; an opinion which is strengthened by the appearance 

 of Goniatites chronologically intermediate between the other 

 two. In order to show the extreme difference which exists 

 between the Nautilus and the Goniatites, M. Barrande has 

 studied the characters of the initial shell in these two genera — 

 a study which has acquired great importance since the publica- 

 tion of Mr. Alpheus Hyatt's " Fossil Cephalopoda." 



Mr. Hj^att has shown that the initial chamber of Nautilus 

 Pompilius shows an elongated nearly linear cicatrice, enclosed 

 by an elliptical surface slightly depressed. He supposes that 

 the ovisack was attached to the elliptic surface, and that the 

 cicatrice 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 has lost its ovisack. 



In Ammonites and Goniatites 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 

 Nautilides or the Ammonides and Goniatides. 



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 

 by 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- 

 tidse and Ammonitidse, they always show a typical ovisack ; it is 

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

 posed by some developmentalists. This difference has induced 

 M. Munier-Chalmas (Comptes Bendus, Dec. 29th, 18t3) to 



* " C6phalopodes, Etudes G^n^rales," 224-230, 1877. 



