126 MR. OWEN’S DESCRIPTIONS OF 
orders, according to the number of cephalic appendages, the presence or absence of pal- 
lial fins, and the connexions of the mantle to the neck. 
The same principles are adopted in the present classification of the Cephalopods in our 
National Museum ; a third order being added to the Octopoda and Decapoda of Leach, 
corresponding with the Polythalamacea of M. De Blainville, and characterised, according 
to the structure of the Nautilus Pompilius, by many short arms destitute of suckers. 
Weigman, also, in his Handbuch der Zoologie, (1832,) makes each of the two divi- 
sions of Leach’s Cephalopods equivalent to the Polythalamacea of De Blainville, which 
he terms Nautilacea. 
But the general organization of the Octopodous and Decapodous Cephalopods, and espe- 
cially their respiratory and circulating systems, correspond so closely, and both at the same 
time deviate so widely from the condition of the corresponding systems in the genus Nau- 
tilus, that the inequality of the value of the three primary divisions of the Cephalopods 
adopted in the synopsis of the British Museum must be obvious: characters, moreover, 
taken from modifications of the locomotive and prehensile organs alone, or associated 
with such minor particulars of organic structure as are applied by Dr. Leach in his 
subdivision of the Naked Cephalopods, can only be viewed as indicative of secondary 
subdivisions of the class. 
A mature consideration of the relations subsisting between the modifications of the 
Cephalopodic type of structure presented by the Pearly Nautilus, and the siphonated 
chambered shell, has led me to perceive that the presence of a siphonated chambered shell 
of itself is not a character of ordinal importance: the organic conditions which may 
justly be regarded as indicating ordinal distinctions relate rather to the amount of deve- 
lopment of the chambered shell, and to its relative position, either as protecting, or 
protected by, the soft parts of its fabricator. Where the chambered shell is limited to 
its hydrostatical functions, and is buried, like an air-bladder, in the interior of the Ce- 
phalopod, and is no longer subservient to its defence, we may infer that an ink-bag will 
be superadded to compensate for the absence of a large defensive case ; and, at the same 
time, that the relief from the incumbrance of a shell so developed will be accompanied 
by an increase of locomotive powers, demanding those modifications of the respiratory 
and circulating functions which are undoubtedly of ordinal importance. 
Now as the Nautilus Pompilius presents an inferior or subdivided type of the respi- 
ratory organs, and as the function of respiration in this species has not the advantage 
of those superadded hearts for accelerating the course of the venous blood through the 
gills which the naked and more active Cephalopods’ possess, and as these most inter- 
esting physiological modifications are related to the size and external position of the 
shell, I feel myself justified in grouping with the family represented by this existing Si- 
phoniferous species, the extinct Orthoceratites, Ammonites, and all other Siphonifera of 
which the soft parts were, in like manner, contained in and protected by a chambered 
shell. To the group thus characterised I have applied the term Tetrabranchiata, derived 
