256 
NAUTILOIDEA. 
flattened sides and a ver}’ broadly rounded periphery, slightly 
flattened in the adult shell. Greatest width of the whorls about 
midway between the umbilicus and the median line of the peri- 
phery. Umbilicus small but distinct. Aperture very wide, scmi- 
lunate, with rounded lateral margins. Septa approximate, very 
slightly sinuous on the sides of the shell, and forming an incon- 
spicuous sinus on the periphery, Siphuncle situated below the 
centre of the septa. Test in the young shell very slightly striated 
transversely, but in the adult covered with very strong, regular, 
prominent, separate ribs or plications, which form a sigmoid curve 
on the sides of the shell, and rather a deep sinus on the periphery. 
The ribs bifurcate, and even trifurcate, in some places on the sides 
of the shell ; the interspaces dividing them are about one half the 
width of the ribs themselves. The ribs mark the cast with very 
conspicuous plications ; the former become flattened by weathering, 
but where well preserved they are seen to be distinctly rounded. 
Itemarl's. This species was differentiated by d’Orbigny from 
Nautilus ehjans, with which it had always been confounded ; it is 
readily distinguished from that species by its much thicker and 
more robust form, closer septa, the position of its siphuncle, and 
the coarser character of its ornaments. It is also met with at a 
lower horizon, both in England and France, than Sowerby’s species. 
From N. Atlas it differs in its broader periphery, open umbilicus, 
closer septa, and position of its siphuncle, which is near the inner 
margin of the septa, instead of the outer, as in N. Atlas. The 
ornamentation of the test is also much finer in the latter than it is 
in the present species. The N. pseucloelecfans referred to by English 
geologists is probably that of 8harpe, which is shown below (p. 273) 
to be Sowerby’s N. elegans. 
According to MM. Pictet and Campiche ^ N. pseudoelegans pre- 
sents some variations in the Sainte-Croix specimens, those authors 
distinguishing four distinct types, of which they give figures 
(plates xiv. & xiv. bis of the work cited above). The first of these 
types, which is from Tonne, is remarkable for the smoothness of 
the test in the young shell ; the second, from Sainte-Croix, differs 
very little from the first, being only slightly more compressed. The 
third type scarcely differs at all from the two preceding ones. In 
the fourth some slight variations in the position of the siphuncle 
are noticed. It is suggested that the variations seen in the Sainte- 
Croix specimens, chiefly concerning the position of the siphuncle. 
^ Descrip, des Fossiles da Terr. Oret. des Environs de Sainte-Croix, Pal. 
Suisse, 1859, ser. ii. pt. i. pp, 123-128. 
