DISTRIBUTION Oi' THE MOLLUSCA IN TIME. 22o 



separate the two former from the tetrabranchiate or tentacu- 

 liferous Cephalopoda (Nantilidse), and to unite them with the 

 dibranchiate or acetabuliferous group (Spirulidae and Belemni- 

 tidse), 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 badl^^ 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 cmbryogeny 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 embiyo of 

 Argonaut to be provided with a shell in the egg, 



M. Fischer concludes, " Is the doctrine of evolution overthrown 

 by the facts M. Barrande has produced ? I do not think it. He 

 has proved that there existed among the ancient cephalopods 

 two great types, which have continued separate during the entire 

 period of their existence ; but the evolution of each of these 

 types remains extremely probable, and conforms to observations 

 made upon them. In such matters one cannot, in effect, ask 

 more than a probability."* 



'^Jour. de Zool., iv, 419, 1877. 



