NAUriLID^. 
259 
>SV. Char. “ Species near to N. Neckerianas, Pictet, but distin- 
guished by the siphuncie placed on the inner third of the whorls, 
and by a depression in the septum near the return of the spire.” 
{D'Orhi^nif.) 
Remarls. The depression mentioned by d’Orbigny is simply a 
median lobe in the sutures, and is of no specific importance what- 
ever, as it is well known to occur in many species of Nautilas ; 
d’Orbigny’s description is, in fact, very inadequate, and were i^ not 
for his reference to fVi, NecJcerianus { = y. radiatus), it would be 
impossible to identify his species. iV. Alhensis is thus described by 
Pictet and Campiche : — 
“ Shell slightly inflated in the young, but more so in the adult ; 
the periphery (region externe) rounded. Umbilicus closed, and 
indicated by a simple depression when the test is present ; iiifun- 
dibuliform and narrow in the cast. Aperture (Bouchel higher than 
wide, if the entire height be taken into consideration ; its width in 
the median line is | greater than its height. Septa rather sinuous, 
convex forwards in the outer third of the sides, and backwards in 
the inner third. A small depression [median lobe] is observed in 
the young shell, in the centre of the basal margin of the septa ; but 
this depression does not persist, for it disappears in the adult. 
Siphuncie placed in the internal third of the septa. The orna- 
ments consist of ridges which are strongly arched forwards, and 
form upon the periphery a backwardly directed sinus whose sides 
make an angle of about 120°. The ridges become narrow’er, and 
finally almost obsolete around the umbilicus ; they disappear in the 
cast towards the middle of the sides, so that the surface in the 
middle of the shell is nearly smooth. The ridges are thick and 
somewhat wide apart, as not more than about ten, as a rule, or, at 
most, fifteen, can be counted in half the diameter of the shell along 
the median line of the periphery.” 
MM. Pictet and Campiche state further that it is impossible to 
affirm positively that their shell is the N. Alhensis of d’Orbigny, 
the latter having been insufficiently described and not figured. 
However this may be, the Gault of Uolkstone yields abundantly 
a species which, on the whole, agrees very closely with Pictet and 
Campiche’s description and figures of N. Alhensis ; but differs from 
it in respect that, up to a certain stage of growAh, the test is 
smooth, the interval between the smooth and the ribbed part of 
the shell being usually occupied by incipient ribbing. The smooth 
part varies in extent in different individuals. The present species 
differs from N. radiatus (a Lower Greensand species) mainly in the 
imbricating character of the ribs, as well as in their greater coarse- 
ness ; but the two species are evidently closely allied. 
s 2 
