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 N. Bucklandi, 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. 

 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 N. imperialis 

 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. centralis as 

 N. australis. 



The N. 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. Nautilus Sowerbyi. Wether ell. Tab. VI. 



Nautilus Sowerbyi. Weth. 1836. Phil. Mag. and Journ. vol. ix, p. 466. 



— — Morris. 1843. Cat. Brit. Foss. p. 183. 



— — Sow. 1843. Min. Con. vol. vii, p. 35, pi. 627, fig. 1-3. 



— - Sowerbtj. 1849. Dixon's Geol. Hist. &c. p. 121, tab. 14, fig. 28. 



N. Testa lavigatd, lenticulari, ventrali aspectu anguste rofundatd ; umbilicatd, aperturd 

 sub-triangulari ; septisprofunde concavis ; siphone continue-, prope margines dor sales posito, 

 perf oralis ; utroque latere late undulosis et sublobatis ; lobis dorsalibus elongatis, valde 

 reflexis, oblique truncaiis. 



The N. Sowerbyi is an exceedingly well-marked species. It is a smooth, discoidal, 

 convex or rather lenticular shell, somewhat resembling in shape the Dax form of 



