14 FOSSIL MOLLUSC A OF THE CHALK. 



have seen specimens from the Lower Green Sand,* to which Formation M. D'Orbigny 

 states that it is confined in France. 



M. D'Orbigny separated this species from TV! elegans, which it closely resembles ; it is 

 less globose, has broader and more flexuous ribs, deeper septa, and a more distinct 

 umbilicus ; but it is best distinguished by the position of the siphuncle, w^hich in this 

 species is situated between the inner margin and the centre of the septum, and in 

 N. elegans is nearer to the back. Notwithstanding this character, most writers seem 

 disposed to throw them together. 



6. Nautilus radiatus, Sow. Plate V, figs. 1 and 2. 



Nautilus radiatus, Sowerby. Min. Conch., t. 356. 



— — UOrhigny. Paleont. Franc. Terr. Cret., t. xiv. 



N. Testa ovatd, subcompressd, costatd, late umbilicatd ; costis latis, cequalibus, 

 Jtexuosis ; septis arcuatis, margine Jlexuosis, ad ventrem depressis ; siphunculo inter 

 ventrem et septorum centrum posiio. 



Shell gibbose, with sloping flattish sides and rounded back, ornamented with broad 

 iiexuous equal ribs ; septa, with a flexuous margin low at the sides and produced at the 

 back, marked with a deep depression close to the edge of the preceding whorl ; siphuncle 

 between the centre and the ventral margin ; aperture semi-oval, considerably higher than 

 broad, slightly indented by the preceding whorl ; umbilicus large, exposing all the inner 

 whorls, t 



Diameter 7 inches, breadth 4 inches. 



Common in the Chalk with siliceous grains at Chardstock in Somersetshire, and 

 also found in the Grey Chalk near Dover, Guildford, &c. Li Professor Forbes's ' List of 

 Lower Green Sand Fossils' it is mentioned as found at Atherfield and Sandgate, and 

 M. D'Orbigny quotes it in France from the Lower Chalk, the Upper and Lower Green 

 Sands. 



N. radiatus is easily distinguished from N. pseudo-elegans by its large umbilicus, the 

 flexuous margin, and the ventral depression of the septa ; the latter character separates it 

 also from N. Neocomiensis. 



* Journal of Geol. Society, vol. i, p. 353. 



f The ribs begin to show themselves when the shell is about one inch in diameter ; previously, it is 

 ornamented with fine sharp elevated lines marking the growth of the shell, crossed by finer longitudinal 

 lines, as seen in fig. 2 ; a similar ornament is found in many species of Nautilus when young. 



