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 vela 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 Nautilus, 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 Nautilus 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 nidus ? 

 * MM. de Blainville and d'Orbigny have founded on these remains two genera, which they have named 

 Conchorhjncus and Rhyncoteuthis. The reasons advanced for supposing that the Rhyncolites were not the 

 mandibles of any of the Nautilidae or Ammonitidae 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. 



