76 BULLETIN OF THE 



cast, and the sections of N. lineatus, which I have seen, indicate that it 

 is present. One of these casts has a very thick tumid shell over the 

 apical portion, and the other, which has a thin shell, exhibits a trans- 

 verse depression just inside of the siphonal ccecura. This shows that 

 the shell not only differs in thickness on the apex, but is more or less 

 corrugated also, as if by a scar. 



The oval outline of the area of the cicatrix, slightly flattened on the 

 ventral side, is singularly like the adult of the Nautilus Bohemicus, 

 and others of the Silurian Nautili described by Barrande.* The 

 regular ellipse of the young of the latter, and the flatter cone of the 

 young of the Carboniferous Nautili, is not represented at all in the 

 young of N. lineatus and N. atratus of the Jura. 



UMBILICUS. 



As the embryo of the typical Ammonites, and the closely coiled 

 Goniatites, such as G. diadema, approaches the beginning of the first 

 whorl, its flattened dorsum becomes depressed or concave on either side 

 as previously described, and when the apex of the first whorl bends 

 dorsally, hollows are formed on either side, closed at the centre. These 

 are the embryonal umbilici. 



They do not exist in Nautilus, but their homologues are probably 

 found in the ovisac, as previously pointed out, though they can form no 

 umbilici properly so called. The close coiling of the first whorl forms 

 the umbilici by enclosing these spaces in Goniatites and Ammonites. 

 In Nautilus they can never be so enclosed, owing to the loose coiling 

 of the first whorl. The umbilicus of Nautilus Pompilius penetrates 

 entirely through the whorls, as it does in the group of Nautilini among 

 the Goniatites, where the ovisac does not fill up the centre. The 

 lateral depressions or sinuosities observable on the sides of the outer 

 rim of the scar in N. Pompilius, the homologues of the umbilici of 

 Ammonites, were not observable in N. atratus, since, as previously 

 stated, no well-defined scar was observable among the fossils which 

 came under my observation. 



WHORLS. 



The whorl of the typical Ammonites, and the closely coiled Goni- 

 atites is at first as broad as the ovisac, then rapidly contracts, becoming 



* Plate IV, Fig. 6. 



