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young shell. This fact is very interesting since here we also find a 

 high degree of acceleration in other characters. Thus the dorsal 

 furrow appears in the ananepionic substage at a considerable dis- 

 tance from the gyroceran bend and continues after this, as shown in 

 Fig. 20, along the dorsum and is continuous with that of the para- 

 nepionic. 



The costations appear in the neanic stage. 



Cymatoceras Deslonchampsianum. 



Nautilus Deslonchampsianus, D'Orb. {Terr. Jurass., PL xx). 



Loc, Rouen, France, Cretaceous. 



PI. xii, Figs. 22-27. 



This species, which is represented by several good specimens of 

 the young, has very nearly the same ontogeny as Cymatoceras 

 elegans, except, of course, in the specific characteristics and the 

 position of the siphuncle. This last is propiodorsan in the meta- 

 nepionic instead of being propioventran as in C. elegans (?). The 

 sutures of the early epembryonic stages differ from those figured by 

 Branco for the same species, but this may be owing to the fact that 

 we have really observed different species. The dorsal furrow 

 appears as in Cymatoceras elegans in the metanepionic at the second 

 septum, as in Fig. 24, and is continued in the paranepionic sub- 

 stage. 



Cymatoceras simplex (?). 



Nautilus simplex, Sow. (Mm. Conch., PL 122). 



Loc, England, Cretaceous. 



PL xii, Fig. 28. 



This single specimen differs somewhat from the specimens of 

 Cymatoceras deslonchampsianus and may be more distinct in the 

 adult, but I do not feel sure of the fact that it is a different species. 



It has been figured in this connection because it shows that the 

 dorsal furrow is present in this shell both in the metanepionic and 

 paranepionic substages. 



Cymatoceras radiatum. 



Nautilus radiatus, Sow. (Mm. Conch., PL 356). 



Nautilus radiatus, D'Orb. (Terr. Jurass., PL xiv). 



Loc, Rouen, Cretaceous. 



PL xii, Figs. 29 and 30, and PL xiii, Figs. 1 and 2. 



The ananeanic substage is shown in outline in centre of Fig. 30, 



