90 BULLETIN OF THE 



shallow depression in the edges of the suture being the only remnant 

 of the dorsal lobe. 



The second and succeeding septa of the young Nautilus Pompilius 

 differ in the possession of a narrow ventral cell upon the median line. 

 This subsequently loses its prominence, but is not wholly lost in the 

 general expansion of the cell ; even in the adult there is a slight rising 

 of the suture as it approaches the median line of the abdomen on 

 either side. The first septa of Nautilus Pompilius, as might be supposed 

 from the form of the whorl, are remarkably broad from side to side, 

 and slightly inflected on the dorsum on either side of the posterior in- 

 flection or deep dorsal lobe previously described. 



In the Clymenia3 the dorsal lobe exists, and it may be observed at 

 an early stage, very large and distinct even in the simple sutures of 

 Clymenia laevigata. Sandberger's researches show that it is a common 

 characteristic of all the species of this Devonian group. The siphonal 

 funnel of the Clymenia?, which occupies the bottom of this lobe, is 

 evidently entirely independent of the lobe itself, as in the case of the 

 ventral lobe of Goniatites and Ammonites. The homology of the 

 dorsal lobe in the young Nautilus with that of the adult Clymeniae, can 

 hardly, therefore, be considered doubtful. It occupies the same position, 

 and has a similar conical form, with an accompanying broad, deep, and 

 rounded sutural inflection. This is especially marked in the Aganites 

 group of the Tertiary, where the siphon is variable in position, and 

 sometimes close to the dorsum occupying the area of the dorsal lobe, as 

 in Nautilus zic-zac, and in other species more central. Here we have 

 an additional reason, besides that previously found in the earlier 

 occurrence of the Goniatites, for separating the Clymenia? including also 

 Aganites as a distinct genetic series from the Ammonoids, notwith- 

 standing the ventral lobe of Clymenia pseudogoniatites. 



The young Nautili of the Silurian, as figured by Barrande, have 

 shallow concave septa, and a more or less shallow outline in accordance 

 with the elliptical or circular shape of a section of the young. This is 

 especially noticeable in the figure of Nautilus ellipticus by Barrande.* 



At a more advanced age all the species except Nautilus vetustus have 

 an elliptical outline more or less flattened on the abdomen. They also 

 possess, in the adult, a narrow ventral cell, like that of the young Nau- 

 tilus Pompilius, on the third and succeeding septa. The Carboniferous 



* Op, cit., Plate XXXIT, Fig. 1. 



