CEPHALOPODA. I , 



A flat discoidal shell, rounded on the ventral aspect, and presenting obscure 

 undulations similar to those which characterise N. regalia. The aperture has 

 an elongated, subquadrate shape ; the umbilicus is narrow ; the septa concave, 

 and slightly undulated ; they present on each side, in a line with the preceding- 

 whorl, a slight depression, which appears to be the first indication of the lateral 

 lobes so fully developed in the N. Parkimoni ; the siphuncle is excentric ap- 

 proaching the dorsal margin ; the dorsal lobes arc short, very slightly concave, 

 obliquely truncated, and not recurved. The lines of growth are prominent, and 

 decussated more strongly than those of the two preceding species, and their undula- 

 tions are broad and shallow. 



The Nautilus urbanus is distinguishable from N. centralis by its flatness, and the 

 greater length of its aperture ; and from N. regalis by its open umbilicus, the 

 truncated extremities of the dorsal lobes of the septa, and its discoidal shape. It is 

 a very rare shell. The figures 2a, 2b, Tab, III, are taken from the shells drawn in 

 ' Mineral Conchology,' the only specimens with which I am acquainted. The larger 

 one, belonging to Mr. Sowerby, was found in excavating St. Katharine's Docks, 

 near the Tower of London ; the smaller one forms part of Mr. Bowerbank's 

 collection, and was obtained from Sheppy. 



The size of the larger individual figured is 7'4 in. in diameter, by 3*4 in. Fig. 4, 

 Tab. VIII, represents the septum. 



No. 10. Nautilus imperialis. Sowerby. Tab. V. 



Nautilus imperialis. J. Sow. 1812. Min. Con. vol. i, p. 9, tab. 1, upper and right-hand 



figures. 



— — Defrance. 1825. Diet, des Sci. Nat., tome xxxiv, p. 297. 



— — Wether. 1836. Philos. Mag. and Journ. vol. ix, p. 465. 



— Bucklandi. Michelotti. 1840. Indice ragionato, &c. p. 4. 



— imperialis. Morris. 1843. Cat. Brit. Foss. p. 182. 



— — Pictet. 1845. Traite element, de Paleont. vol. ii, p. 338. 

 — Sowerby. 1849. Dixon's Geol. Hist. &c. pp. 110, 120. 



N. Testa spharoidale ; umbilicatd, umbilicis angustis, profundis ; aperturd sub- 

 ellipticd, semihmari ; septis undatis, siphone mediocri dorso-excentrali perforatis ; lobis 

 dorsalibus I at is et perparum rejlexis. 



This species is easily distinguished from the N. centralis by the excentric position 

 of the siphuncle, as well as by the broad and reflected extremities of the dorsal lobes, 

 which form, as it were, an axis to the shell. Its orbicular form, the lunate shape of 

 the septa, and the recurved dorsal lobes, distinguish it as clearly from N. regalis and 

 N. urbanus. 



