AA EOCENE MOLLUSCA. 
which are bent forwards. The chambers are separated by transverse partitions, more 
or less undulated ; and in one species, VV. Parkinsoni, they are distinguished by lateral 
angular lobes, resembling those of Aturia (Nautilus) zic-xac, and the margins are 
invariably simple and entire. The discs of the septa are perforated at the centre, or 
at parts more or less distant from the margins, but never at the margin, by a calcareous 
siphuncle, variable in size and generally discontinuous, that is, extending more or less 
into the preceding chamber, but not into the preceding siphuncular aperture. The 
chambers themselves increase in size to the last, which is sufficiently large to contain 
the whole of the animal; but the ratio of increase is apparently uncertain, and is 
influenced probably by the growth of the animal, which would, of course, depend on 
the supply of food and other circumstances. 
The fossil substances termed R/yncolites, which occur so frequently in the older 
formations, and which are generally believed to be the mandibles of some of the Tetra- 
branchiate Cephalopods, with whose remains they are associated, have been found 
both in the Paris basin and in the tertiary formations in Belgium; but I believe that 
as yet they have not been found in the Eocene strata of England. 
The specific characters in this genus are taken from the curvature of the septa, 
the general outward form of the shell, (which, in fact, determines the shape of the 
septum,) the position of the siphuncle and the condition of the umbilicus. With 
respect to the terms dorsal and ventral, it must be borne in mind that they are used in 
the following descriptions in a sense directly the reverse of that in which they have 
been generally applied. The Nautilus, in its normal position, rests upon, or creeps 
along the ground by means of, the free and expanded anterior portion of the mantle. 
In this position the back of the animal is against the penultimate whorl of the shell, 
and the ventral part is contained within the concavity of the dwelling-chamber. In 
the following descriptions, therefore, the term dorsa/ is used to designate the parts 
contiguous to the penultimate volution of the shell, and which have been generally, 
though incorrectly, described as ventral; and the term veztra/, on the other hand, will 
be applied to those parts on which the belly of the animal rested, and which hitherto 
have usually been termed dorsal. 
At present six species have been found in the tertiary strata of England, and they 
are confined to the older Eocene deposits. In the contemporaneous strata of the Paris 
basin two species occur, one of which is also found in Belgium ; but not either of 
them has as yet been found in England; and four species have been described by 
Sismonda and Michelotti, as occurring in the Miocene formations in Piedmont. Two 
of these last species are referred by those authors to existing species; but the 
accuracy of the identification is questioned. 
