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The Shell. — The Nautilus is the only living Cephalopod 
which can be said to possess a shell homologous to the true 
molluscan shell ; it is true that the Argonaut is provided 
with a beautiful thin, papery, translucent, involuted shell, 
but this is secreted by the broad web-like extremities of the 
anterior pair of arms of the female only, who uses the unoccu- 
pied spiral portion of the shell as a nidemental chamber for 
the safe deposition of her eggs. The shell, unlike that of 
any other mollusc, is not attached to the animal by any 
muscular connection. 
The true type of a Tetrabranchiate shell is furnished by 
the Pearly Nautilus, the only surviving representative of a 
great number of genera, and thousands of species which once 
swarmed in the seas of the palaeozoic and secondary epochs, 
and whose fossilised remains abound in all formations from 
the Silurian to the Cretaceous epoch, one genus alone having 
been continued through the tertiary age to the present day. 
The shell of the Nautilus is involuted, and divided by 
transverse partitions (septa) into numerous chambers, each of 
which has in its turn been inhabited by the owner; each 
septum is curved with its concave side turned towards the last 
formed chamber, and all the partitions are pierced centrally 
by a tube (siphuncle) ; the edges of the septa (sutures) are 
simple. The inner layers of the shell are nacreous or pearly, 
whilst the outer ones are porcellanous. 
The fossil genera, which are numerous, agree with the 
Nautilus in having external chambered shells, with their septa 
pierced by a siphuncle ; they differ from it in the shape and 
proportions of the shell, the position of the siphuncle, and 
the markings of the sutures. In the genera Orthoceras and 
Baculites the shell is straight ; in Trochoceras and Turrilites 
it is spiral ; in Ascoceras and Ptychoceras it is bent on itself ; 
in Cyrtoceras and Toxoceras it is curved ; whilst in Gyro- 
ceras and Crioceras it is discoidal. 
