16 FOSSIL MOLT.USCA OF THE CHALK. 



2^ inches in diameter and 1^ inch broad at the mouth ; it is interesting, as it shows 

 the change from the neatness of the young shell to the coarse irregular wavy form of the 

 adult. 



This shell is found more plentifully in the upper part of the Lower Green Sand, where 

 it reaches a diameter of 4 or 5 inches. M. D'Orbigny has not met with it in France. 



When young, N. undulatus has considerable resemblance to N. largilliertianuH, from 

 which it is distinguished by its closed umbilicus ; in its undulated stage it cannot be 

 confounded with any other species. 



9. Nautilus largillieutianus, D' Orh. Plate VI, figs. 1 and 2. 



Nautilus labgilliertianus, D'Orbigny. Paleont. Fran9. Terr. Cret., t. xviii. 



N. Testa discoided, compressd, umbilicatd, delicatim striata; lateribus complanatis ; 

 dorso planiusculo, in medio lineato ; wnbilico magno, margine declivi ; aperturd ovato- 

 rhomhoided ; septorum marginibus flexuosissimus ; siphunculo sub-ventrali. 



Shell discoidal, with fine lines of growth which are not seen in the cast; with flat 

 sides and a somewhat flattened back, marked with a slightly raised mesial line ; umbilicus 

 large, wdth perpendicular sides, exposing all the inner whorls ; aperture rhomboidal 

 with rounded corners, deeply indented by the preceding whorl ; margin of the septa very 

 flexuous ; siphuncle very near to the inner margin. In old shells the back is slightly 

 undulated. 



Diameter 2 inches, breadth -Iths of an inch. 



This elegant shell is easily distinguished from the other Cretaceous species by the 

 rhomboidal form of the whorls, well-defined umbilicus, and flexuous septa ; the striae are 

 only seen in very well-preserved specimens. 



Found occasionally in the Grey Chalk at Lewes, and in the Chloritic Marl of the 

 Isle of Wight ; more common in the Chalk with siliceous grains of Chardstock and 

 Chaldon. 



M. D'Orbigny cites it as common in the Lower Chalk of the North of France. 



10. Nautilus fleuriausianus, B'Orb. Plate VI, fig. 3. 



Nautilus fleuriausianus, D'Orbigny. Paleont. Fran?. Terr. Cret., t. xv. 

 Nautilus Sowerbyanus? D'Orbigny. Ibid., t. xvi. 



N. Testa lateribus compressd, dorso rotundatd, sub-umbilicatd, lavi ; aperiurd semi- 

 ovatd ; septorum marginibus Jlexuosis ; umbilico mifiimo ; siphunculo sub-centrali. 



Shell with sloping and somewhat flattened sides and rounded back, smooth, with a 

 very small umbilicus ; mouth semi-oval, higher than wide, deeply indented by the pre- 



