NAUTILUS. 15 



7. Nautilus Neocomiensis, B'Orb. Plate V, fig. 8. 



Nautilus Neocomiexsis, B'Orbigny. Paleont. Fran9. Terr. Cret., t. xi. 



N. Testa injlatd, lateraliter cotnpressd, late iimbilicatd, costatd; costis latisjlexuosis, 

 dorso suh-angulatis ; septis marline paulum Jlexuosis ; siplmnculo inter ventrem et septorum 

 centrum posito. 



Shell gibbous, with flattened sides and a broad umbilicus, ornamented with rounded 

 flexuous ribs, which are less marked at the sides but broad and large on the back, where 

 they bend so rapidly as to form an almost angular sinus ; the sides of the whorls being 

 nearly parallel, and the whorls increasing but slowly in breadth, the whole shell has a 

 flattened form by which it is easily recognised ; margins of the septa slightly curved ; 

 siphuncle half-way between the centre and the inner margin of the septa. 



We have only seen one small specimen from the Grey Chalk, which we can refer to 

 this species ; it is from Urchfont near Devizes, in the Collection of i\Ir. Cunnington, and is 

 2 inches in its greatest diameter and \\ inch in breadth; although a good deal broken, 

 it shows all the peculiar characters of the species. This species is more common in the 

 Lower Green Sand of Dorking, Atlierfield, Saudgate, &c., where it sometimes reaches 

 above 7 inches in diameter. M. D'Orbigny quotes it as common in the lower beds of the 

 Middle Division of the Neocomian Formation in France. 



The only species with which this can be confounded is N. radiatus, from which it may 

 be known by its flatter form, larger umbilicus, septa less flexuous and without any de- 

 pression at the ventral margin, and the more angular sinus of the ribs on the back. 



8. Nautilus undulatus, Sow. Plate V, fig. 4. 



Nautilus undulatus, Sowerby. Mineral Coiichology, t. xl. 



N. Testd gihhosd ; junior e l(Em,laterihu8 suh-compressis, dorso lato-rotundato ; adultd 

 lateraliter exjjansd, transversim undulato-costatd ; septorum margine Jlexuoso ; siplmnculo 

 sub-ventrali ; aperturd semi-ovali ; umbilico clauso. 



Shell when young, smooth and regular, with flattish sides, a broad rounded back, and 

 semi-oval aperture ; when about two inches in diameter, it enlarges rapidly, expands at the 

 sides, and begins to undulate over the back ; when adult, the whole shell is ribbed in 

 broad, coarse undulations, which are deepest on the back, but ill-defined on the sides ; 

 back marked by a line at first shghtly raised, which in the old shell forms a ridge 

 between the undulations ; umbilicus covered ; septa flexuous ; siphuncle near the ventral 

 margin. 



We have only seen one specimen of this species from the Chalk, which was found by 

 Mr. Wiest in the Chalk with siliceous grains of Chardstock; it is a young shell. 



