XAUT1LIDJ2. 
203 
S/>. Char. “ Shell moderately inflated, umbilicus not very large, 
■whorls flattened both on the sides and on the perijDheiy, the greatest 
thickness being at the umbilicus. The sides of the latter are steep 
and the borders subaugular, the test being here very thick. The 
septa, of -which there are about thirteen in an entire whorl, form a 
strongly marked sigmoidal curve upon the sides of the shell, but are 
Fig. 35. 
Isautilus Jourdani. — a, lateral view, showing the deep umbilicus and the orna- 
ments of the test ; h, peripheral view, showing the form of the sutures. 
Drawn from a specimen in the British Museum (No. 19587). About one 
half natural size. 
only very slightly sinuous on the periphery. The siphuncle is situ- 
ated a little below the centre. The test is covered with numerous, 
thread-like, longitudinal ridges, two or three in the space of 1 line, 
more approximate on the periphery than on the sides, as is usually 
the case with such ornaments. The ridges become partly obsolete 
when the shell has attained a certain diameter, say between 4 and 5 
inches. Fine lines of growth are seen where the shell is well pre- 
served, especially in the region of the umbilicus. 
“ It should be added that the interior of the umbilicus is orna- 
mented with fine, radiating, flexuous ridges directed forwards ; these 
are crossed by spiral ridges somewhat widely spaced. 
RemarJi's. “ This species is distinguished from N. ornatus (to be 
subsequently described), which appears to be its nearest ally, by its 
