14 EOCENE MOLLUSCA. 



required in the nonplicated shell, for the preservation of an animal, whose habitation, for 

 the most part, is at a considerable depth, where the pressure of water is much increased, 

 than in the plicated species, the peculiar construction of which would afford sufficient 

 resistance, without that additional support which the smoother species may receive 

 from this singular structure of the mantle. If, however, a necessity exist for the 

 preservation of the shell in ordinary cases, how much more essential would it be that 

 some compensating power should be possessed by an animal whose existence, in all 

 probability, is dependent upon the buoyant principle of its partitioned shell ; and how 

 probable does it appear that this, an ordinary provision, should be employed for its 

 protection. 



The tubular character of the siphuncle suggests an hydraulic action. To explain 

 this, it is necessary to invest the animal with the power of emptying and filling the tube 

 at discretion ; and this power it may be presumed to derive from the pressure upon the 

 pericardial cavity, caused by the folding and contracting within the shell of the large 

 cephalic mass. Under this pressure the fluid would be injected the whole length of 

 the siphuncle, and, on the removal of the pressure, would return into the pericardium, 

 to be there renovated and vivified with the other fluids, to be again injected when the 

 animal returns within the shell. If the siphuncle had been a solid body, or composed 

 of muscles, fibres, &c, it would have required to be permeated with arteries, blood- 

 vessels, &c, for its sustenance ; but by the simple process of the fluid returning into 

 the body of the animal, all the complicated apparatus necessary to sustain a fleshy 

 body is superseded ; circulation and renovation are accomplished, and the fluid is thus 

 maintained in a condition capable of affording the nourishment to the shell which the 

 present hypothesis requires. 



The theories here suggested are, as all other theories on the same subject must 

 for the present be, merely speculative ; for, to quote the observation of Professor 

 Owen,* " much remains to be done before the theory of the chambers and siphuncle 

 can rest on the sound basis of experiment and observation." These alone will 

 satisfactorily determine the real purposes of the membranous siphuncle ; but, for my 

 own part, I believe that the primary, and probably the only, function of that organ is to 

 maintain the vitality of the shell, and that it may be looked upon as an elongated 

 caecum ; and that it is not, under any circumstances, used by the animal as a hydro- 

 static balance. 



It is unnecessary here to particularise the various forms of external shells presented 

 by the extinct tetrabranchiate Cephalopods, inasmuch as, of the numerous genera 

 which swarmed in the ancient seas, only the Nautilus survived the secondary period. 



The dibranchiate Cephalopods, with the exception of the genus Argonauta (which, 

 with Bcllcrojjhon, constitutes Professor Owen's family of testaceous Octopods), are without 



* Memoir on the Nautilus Pompilius, p. 47. 



