48 EOCENE MOLLUSCA. 
The Nautilus imperialis is a somewhat globose shell, rather narrow on the ventral 
aspect, whence the aperture assumes a sub-elliptical form; the umbilicus is small and 
deep. It is well displayed in the large figure, Tab. V, taken from a specimen in Mr. 
Bowerbank’s collection, but generally, 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 side of the centre of the 
disc. It appears to vary in its position, gradually becoming more distant from the 
dorsal margin as the shell enlarges. The lines of growth are reflected 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 WV. Buchklandi, 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 
NV. centralis as well as with NV. imperialis. Whether the alleged identification of JV. 
Bucklandi 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 JV. cmperialis 
did not appear to differ from JV. centralis, and Michelotti has, in fact, relied implicitly 
on that author; he has even copied the mistake made in quoting JV. centralis as 
NV. australis. 
The JV. imperialis attained a very large size; a specimen from Sheppy in the 
Museum of the Geological Society measures 12 inches by 8°75 in. across. It appears 
to have been widely spread, being found at Highgate, Hornsey, Brentford, Sheppy, 
Cuffell near Basingstoke, Clewett’s Green, Newnham, Bognor, and Bracklesham. 
The form of the septum is shown in Tab. VIII, fig. 1. 
No. 11. Nautitus Sowersyl. /Vetherell. Tab. VI. 
Navritus Sowersyl. Weth. 1836. Phil. Mag. and Journ. vol. ix, p. 466. 
— -- Morris. 1843. Cat. Brit. Foss. p. 183. 
—_— _ Sow. 1843. Min. Con. vol. vu, p. 35, pl. 627, fig. 1-3. 
— _— Sowerby. 1849. Dixon’s Geol. Hist. &c. p. 121, tab. 14, fig. 28. 
N. Testa levigatd, lenticulari, ventrali aspectu anguste rotundatd ; umbilicata, apertura 
sub-triangulari ; septis profunde concavis ; siphone continuo, prope margines dorsales posito, 
perforatis ; utroque latere late undulosis et sublobatis ; lobis dorsalibus elongatis, valde 
reflexis, oblique truncatis. 
The WV. Sowerby: is an exceedingly well-marked species. Itis a smooth, discoidal, 
convex or rather lenticular shell, somewhat resembling in shape the Dax form of 
