Mr. J. S. Miller on Belemnites. 49 



durable^ while the fragile texture of the chambered portion has rendered it 

 liable to be destroyed by the least violence ; and consequently this part has been 

 preserved only when its chambers were filled with mineral matter ; and even 

 then a subsequent decomposition of the transverse septa has frequently sepa- 

 rated the series of casts formed within the chambers. 



The chambered structure of the conical shell of the Belemnite, and its siphun- 

 cle^ exhibit a striking analogy with the Orthoceratite, the Nautilus, and the 

 Ammonite. It differs, however, essentially from the Orthoceratite in the cir- 

 cumstance that all parts of its shell present on fracture a laminar fibrous spa- 

 those substance ; in its being provided at its posterior end with a more or less 

 elongated and encompassing guard or sheath ; and in the siphuncle of the 

 chambered cone being situated near the margin. This last character, com- 

 bined with the fibrous and spathose texture of the shell, will at all times, in 

 my opinion, atford a discriminating character between the chambered cone of 

 a Belemnite (even when occurring separated from its guard) and the Ortho- 

 ceratite. 



From this analogy of the Belemnite to the Nautilus in the chambered parts 

 of the shell, it appeared probable that, like the latter, it could be made buoy- 

 ant at the will of the animal. But as my examination of the guard had led me 

 to the conclusion that this part had been at its first formation solid and spa- 

 those as it is at present, a difficulty presented itself, as, although the form 

 and weight of the guard would tend to preserve the animal in an upright po- 

 sition, yet this weight would more than counterbalance the buoyancy which 

 could be obtained by chambered portions no greater than those found in the 

 specimens which are commonly met with. To ascertain the size of the chamber 

 which would be requisite to compensate for the weight of the guard, I made 

 the following experiment. 



I fastened with gum a piece of oiled paper of a conical form in the cavity 

 from which the chambered part had fallen out of a large Belemnite ; and putting 

 into this cone some cotton, to prevent the paper from collapsing by the pres- 

 sure of the water, I found that the apparatus floated with a paper of no larger 

 size than from some known fine specimens we may fairly conclude the outer 

 chamber to have attained. 



Thus far successful, I now resorted to a particular examination of the be- 

 lemnitic shell, to acquire, from its laminar construction and muscular impres- 

 sions, facts to ascertain how far they might confirm or confute my theory, — 

 that it had sustained no change by petrifaction^ but was from its first origin 

 composed of fibrous spar. 



VOL. II. — SECOND SERIES. H 



