NAUTILUS. 13 



Diameter 6^ inches, breadth 5 inches. A gigantic specimen, sent me by Mr. Tyler 

 from Hamsey near Lewes, reaches 11^ inches in its greatest diameter, and the ribbing 

 continues of about the same breadth throughout. This species is common in the Grey 

 Chalk of the South of England and in the lower beds of the Chalk of France. 



For many years all the ribbed Nautih found in the Chalk were confounded together 

 under the name of JV. elegans. Sowerby separated N. radiatus. But we owe to M. 

 D'Orbigny the clear distinction of the different species, which may be known as follows : 



N. elegans is the most globose shell of the group, and has the smallest umbilicus ; it is 

 also to be known by the position of the siphuncle between the back and the centre of 

 the septum. 



N. pseudo-elegans is rather less globose than the preceding, has a more distinct 

 umbilicus, broader and fewer ribs, and the siphuncle between the ventral margin and the 

 centre of the septum ; the two species are so much alike that the only character to be 

 relied on for distinguishing them is the position of the siphuncle. 



N. radiatus is easily recognised by a flatter form, with sloping sides, large umbilicus 

 and deep septa with a sinuous margin ; the siphuncle occupies nearly the same position 

 as in the last ; but there is also a marked depression of the septum close to the ventral 

 margin, which is not found in either N. elegans ox pseudo-elegans. 



N. Neocomiensis has flat and nearly parallel sides, a very large umbilicus, the ribs 

 much bent on the back, the siphuncle between the centre and the ventral margin, but 

 without the ventral depression of the septum. 



N. Deslongchampsianus is easily distinguished by the cross longitudinal ribs, and even 

 if these are lost, by the elevated edge of the umbilicus. 



\w all these species the ribbing is only seen in the exposed portion of the outer whorl, 

 being concealed within by a layer of shell formed by the ventral fold of the mantle. 

 Thus specimens which have lost the outer whorl may appear partially or entirely smooth. 



5. Nautilus pseudo-elegans, D'Orb. Plate IV, fig. 2. 



Nautilus pseudo-elegaxs, D'Orbigny. Paleont. Franc. Terr. Cret., t. viii and ix. 



N. Testa ovatd, subcompressd, costatd, umbilicatd ; costis (Bqualibus, rotundatis, 

 Jlexuosis ; umbilico parvo ; aperturd senii-ovali ; septis arcuatis ; siplmnculo inter ventrem 

 et septi centrum posito. 



Shell ovate, ribbed, with a small well-marked umbilicus ; ribs broad, rounded, and 

 flexuous, with a deep sinus on the back ; aperture semi-oval, higher than broad, deeply 

 indented by the preceding whorl ; septa arched, with the siphuncle between the centre 

 and the ventral margin. 



Diameter 7 inches, breadth 4 inches. 



Common in the Grey Chalk of Lewes and elsewhere. Professor E. Forbes appears to 



