322 
XAUTILOIDEA. 
1850. Nautilus imperialism d’Orbigny, Prodr. de Pal^ont. Stratigr. 
vol. ii. p. 338. 
1852. Nautilus imperialis (pars), Giebel, Fauna der Vorwelt, Band iii. 
Abth. i. p. 151. 
1854. Nautilus imperialis, Morris, Cat. of British Fossils, 2nd ed. 
p. 306. 
1872. Nautilus imperialis, Whitaker, Geology of the London Basin, 
Mem. Geol. Surv. of England and Wales, vol. iv. p. 580. 
1890. Nautilus imperialis, Foord and G. C. Crick, Ann. Mag. Nat. 
Hist. ser. 6, vol. v. p. 391. 
Sp. Char. “ The Nautilus imperialis is a somewhat globose shell, 
rather narrow on the ventral aspect, whence the aperture assumes 
a subelliptical form ; the umbilicus is small and deep . . . gene- 
rally it is found open only in young shells ; in the larger specimens 
it is usually filled with pyrites or indurated clay. The septa are 
deeply concave, and present a gentle undulation on each side ; the 
dorsal lobes are very broad, inflected towards the axis, and obliquely 
truncated on the inferior margins. The siphuncle is moderately 
large and excentric, being placed on the dorsal [inner] side of the 
centre of the disc [i. e. below the centre of the septa]. It appears 
to vary in its position, gradually becoming more distant from the 
dorsal margin as the shell enlarges [a change of position which is 
of frequent occurrence in Xautili]. The lines of growth are 
refiected backwards in a deep narrow wave, and in the specimens I 
have seen are not decussated as in the three preceding species. 
“In the shell described by Michelotti under the name N. Bucldandi, 
and with which he has associated the present species, the siphuncle 
is central; and that character is, in fact, the reason assigned by 
him for considering his shell to be identical with N. centralis as 
well as with N. imperialis. Whether the alleged identification of 
N. Bucldandi with N. centralis be correct or not, it is obvious that 
the Piedmontese shell cannot be referred to the present species. 
Defrance states that the N. imperiali.s did not appear to differ from 
N. centralis, and Michelotti has, in fact, relied implicitly on that 
author ; he has even copied the mistake made in quoting N. cen- 
tralis as N. australis.” {Edwards.) 
Remarks. N. imperialis attained a large size ; a specimen from 
the Isle of Sheppey, contained in the Museum of the Geological 
Society of London, has a diameter of 1 foot and a width of 8j 
inches. 
The largest specimen in the British Museum (No. C. 3401) is 
11 inches in diameter. 
This is rather a widely distributed species, as may be seen by 
a reference to the list of localities. 
Horizon. London Clay (Lower Eocene). 
