INHABITING MULTILOCULAR SHELLS. 41 
of those referred to the other two families, and thus requiring a se- 
parate consideration. 
It is found convenient to sub-divide the Nautilacea into four ge- 
nera, characterized by the shape of the shell, which passes, by suc- 
cessive gradations, from a nearly straight cone to the spiral form and 
enveloped whorls of the Nautilus. One reason why we prefer this 
mode of arrangement is, that it will apply, in exactly a similar man- 
ner, to the Ammoneata; and as we shall show the probability there 
is that considerable changes in the animal economy were required 
for these various alterations of shape, there will, we think, appear 
quite sufficient reason for the adoption of this system of classification. 
The four genera which now comprise D’Orbigny’s first family, 
Nautilacea, are Nautilus, Endosiphonites, Spirula, and Orthoceratite. 
It will be remembered that the Mautilacea generally have the siphun- 
cle central or ventral, and septa comparatively simple ; and, bearing 
this in mind, let us proceed to consider the genera sertatim. 
The first is the Nautilus, concerning which we have already said 
so much that it will hardly be necessary to enlarge much further 
upon it.. The chief point to be attended to in the description of the 
shell are three: the usually central position of the siphuncle, the en- 
velopement of the whorls by the last, and the simplicity of the walls 
of separation. There are exceptional species with regard to each of 
these characters, but on the whole they are very constant, and the 
slight departures from the type not more than we see in other and 
more completely: known genera. The number of species now known 
in our seas is only two, while more than fifty fossil ones have been 
determined. 
The recent Nautilus is found in very distant localities, and seems 
to be capable of inhabiting latitudes varying from thirty to forty de- 
grees north to nearly as much south. It has been seen, according to 
navigators, in the Red Sea and the Indian Ocean, while that exa- 
mined by Mr. Owen was taken near the group of islands called the 
New Hebrides, in the South Pacific Ocean. The habits of the ani- 
mal requiring that it should obtain its prey near the bottom of the 
sea, it is not often seen on the surface, and when it does appear is 
not taken without some trouble, as it sinks immediately on the ap- 
proach of danger. 
Of the fossil species referred to this genus, by far the greater 
number occur in the oolitie and carboniferous systems ; in the former 
accompanied by a vast number of Ammonites and other Cephalo- 
pods, which, being all carnivorous, indicate an extreme abundance of 
VOL. IX., NO. XXV. 6 
