8 EOCENE MOLLUSCA. 
as at first was supposed to be the case, but the actual fabricator of the shell ; and it is 
believed that the broad membranes usually termed ve/a or velamenta, into which the 
extremities of the posterior pair of arms are expanded, and which usually envelope 
the shell, are the organs by which the deposition is effected; the mantle itself, 
apparently, not being capable of a calcifying secretion. 
The beaks or mandibles with which the mouths of the Cephalopods are armed, vary 
in structure according to the habits of the animal. In the dibranchiate Cephalopod, 
whose principal food consists of fish, the mandibles are sharp, and entirely composed 
of horn; but, with the tetrabranchiate Cephalopods, the mandibles are blunt, and 
cased at their extremities with hard calcareous matter, adapted for the crushing of 
shells, and the defensive coverings of crustacea. The fossil substances called 
Rhyncolites, resembling the mandibles of the recent Wawtilus, and found associated with 
the numerous chambered shells so abundant in the secondary and transition forma- 
tions, appear to be remains of Ammonites, and the other cognate extinct genera by 
which those shells were inhabited.* 
That the external chambered shells of the Cephalopods act in the same way as the 
swimming bladders of fish, and serve as floats, is obvious from the circumstance that, 
when deserted by the animal, they swim on the surface of the water. To an animal 
seeking protection against its enemies, by an instantaneous sinking in the sea, this 
tendency of the shell to float would prove a serious and dangerous impediment, if the 
animal itself did not possess the means, in some way or other, of increasing on 
the instant its specific gravity; and it has long been the opinion of naturalists 
that the siphuncle is subservient to this purpose, although a difference of opinion 
has prevailed as to the mode of operation. Dr. Hooke, so far back as the 
beginning of the last century, expressed an opinion that the MVawti/vs had the power 
of generating air to fill the deserted chambers, and that by the injection or exhaus- 
tion of this air through the siphuncle, the specific gravity of the shell could be 
diminished or increased. It is ascertained, however, that there is not any communi- 
cation between the siphuncle and the empty chambers; and Mr. Parkinson, who, in his 
‘Outlines of Oryctology,’ adopts an hypothesis similar to Dr. Hooke’s, suggests that the 
tube is elastic and dilated by gaseous or aqueous fluids, the alternation of which 
produces a corresponding change in the specific gravity of the shell. Dr. Buckland 
are found in different stages of growth, and they always exhibit the usual indications of successive periodic 
enlargements. Again, Mr. Adams states, “that it does not appear that the female is able to exist long when 
disengaged from the shell.” How can these facts be reconciled with the theory that the shell is a mere xidus ? 
* MM. de Blainville and d’Orbigny have founded on these remains two genera, which they have named 
Conchorhyncus and Rhyncoteuthis. The reasons advanced for supposing that the Rhyncolites were not the 
mandibles of any of the Nautilidee or Ammonitide already known, are far from conclusive ; and these 
genera can only be regarded as arbitrary, though perhaps convenient, divisions, according to the peculiar 
forms presented by the remains. 
