110 



since, except in the form of the septa, and the disposition of the siphun- 

 culus, there is no concurrence of character. The fossil shells of this 

 genus appear to resemble Spirilla J'ragilis, Larn. Nautilus spirulus, Linn, 

 very closely, except in size. In this respect the difference is so very 

 considerable, the fossil shells being so much larger, as almost to warrant: 

 a specific distinction, founded on this circumstance alone. The fossil 

 shells of this genus have hitherto been named Lituites, from their resem- 

 blance, in form, to a bishop's pastoral staff; but, in these pages, I 

 shall denominate them Spmilites, in concordance with the name of 

 the genus. 



The siphunculus, in the recent specimens of this shell, differs from 

 that of the nautilus, in being formed by one continued shelly tube, or 

 by the smaller end of each anterior one terminating in the larger end of 

 each posterior one ; but whether this is constantly the case or not, in 

 fossil specimens, I am unable to decide ; since, in those specimens, in 

 which I have discovered such appearances as would lead to a suspicion of 

 the siphunculus being partly membranous, I have not been able to ascer- 

 tain whether they were fragments of the spirulites or of orthoceratites. 

 The very great length of the straight part of shell, in the spirulites, 

 will necessarily occasion this ambiguity, until some decidedly distinguish- 

 ing marks of the orthoceratites and of the straight part of the lituites can 

 be discovered. As in the nautilites, so in the spirulites, the situation of 

 the siphunculus, varies in different specimens, and perhaps in those 

 which are in every other respect similar. 



We have seen that, in the nautilus, the siphunculus is partly mem- 

 branous and probably elastic ; whilst, in the shells of this genus, we 

 find that the whole siphunculus is of shell, and consequently unyielding. 

 From this difference of organization, a considerable difference must ne- 

 cessarily exist, as to the influence which the introduction of water or of 

 air must have on the buoyancy of the several shells: a difference 

 depending on the greater quantity admissible in a dilatable than in a rigid 

 tube. A more intimate knowledge of the nature of these shells must he 



