Mr. J. S. Miller on Belemnites. 63 



nana, is composed of radiating crystals of carbonate of lime. This instance is 

 important in illustration of my theory, as showing- the actual existence of such 

 a structure in the shell of a living animal. 



VI. Effects of Decomposition on the Shell of the Belemnite. 



In many shells, such as Nautili, Ammonites, and Trochi, the common cal- 

 careous outer part yields more easily to decomposition than the iridescent 

 nacreous internal coating*. We see instances of this in the thin iridescent 

 flattened scales of Ammonites found at Watchet, which are the remains of 

 partly dissolved shells. 



In the Belemnite, however, it appears that the nacreous portion yielded to 

 decomposition more readily than the spathose parts. 



The proofs of this are to be found in the frequent separation of the guard 

 from the chambered cone. These parts were originally connected by nacreous 

 matter, and it is the decay of this matter which has caused them to separate. 

 The further process of decomposition acting on this cavity, produces a groove 

 along the line between the two portions of the guard. 



The central canal, which on breaking the Belemnite is sometimes found to 

 traverse the axis of the guard, and which has been considered by some writers 

 as a channel for nerves or vessels, and by others as a continuation of the 

 siphuncle, is also to be attributed to decomposition. 



There is no connexion whatever between this channel and the chambered 

 portion, which is completely invested and surrounded by the inclosing shell. 

 The manner of its formation appears to me to be, that the nacreous layer 

 which is deposited between each of the spathose ones, is continued at the 

 apex of the guard into a prolonged point, so as to reach the succeeding na- 

 creous layer ; and this substance having decayed while the spathose part re- 

 mained unaltered, a continuous channel has been formed. 



Decomposition also shows itself sometimes in removing the calcareous matter, 

 cementing together the longitudinal portions of the guard, and sometimes we 

 find that particular layers of the calcareous matter are attacked (perhaps from 

 some peculiarity in their composition making them more liable to decay) : and 

 thus a separation takes place between the outer portion of the series of layers 

 and those within the decayed layer. The inner portion is what at an earlier 

 stage of growth was the whole of the belemnitic guard. The outer portion has, 



* This nacre may be better calculated to resist decay, on account of its containing (according 

 to Mr. Hatchett's experiments) 24 per cent of animal membrane, possessing properties similar 

 to coagulated albumen. We have no means of ascertaining what proportion of such substance 

 existed in the nacre of the Belemnite. 



