Classification of Ammonites. ^^ 453 



nearest the outside (it forms the most important part of the 

 sliell properly so called, and M. Siiess has named it ostracum)^ 

 and an internal nacreous layer formed of very small cells, 

 which constitutes the septa and lines the inner surface of the 

 ostracum. The forme)- is secreted by the mantle ; the latter hy 

 the hody of the animal.^^ 



Thirdly, he states, " The whole animal (of the Nautilus), 

 the posterior part excepted, is therefore united to the shell, 

 and the chamber is hermetically closed;" and goes on to say, 

 " the mantle extends in front of this attaching ring {Haft- 

 riufj) ; it is composed of two parts — one, which is very short, 

 corresponding to the antisiphonal region of the animal ; the 

 other, which is much longer, corresponds to the siphonal 

 region, and secretes the shell, with which it is connected by 

 its outer margin." 



These observations come within my study ; and I may 

 observe that they are directly at variance with all my expe- 

 rience in the structui'e and growth of the shells or opercula 

 of Mollusca, and appear to me only to be compared to the 

 Swiss author, living in the centre of Europe, who described 

 ships being built of brick. 



The Nautilus-^\\Q\\ is composed of two layers, the outer 

 chalky and opaque, the inner pearly : the outer is first formed, 

 and forms the edge of the shell ; the inner pearly layer is 

 deposited on the inside of the outer as it is enlarged, the two 

 going on pari passu ', and both are deposited by the mantle of 

 the animal, as all shells are deposited, and as may be seen both 

 in the univalve Turbo and the bivalve Unlones or Aviculcej 

 which exactly agree with Nautilus in structure ; and I should 

 like to know how the body of the bivalve got out of the 

 large mantle to deposit the pearly layer of the inner surface 

 of the shell, which is quite out of its reach and influence. It 

 is quite a new fact to me that the whole animal of a mollusk 

 should be united to the shell, and so hermetically close it ; if 

 true, it would require an entire change in the definition of 

 Mollusca, which are always entirely free from the shell, and 

 only attached to it by ])eculiar muscles ; and I can vouch for 

 this being the case in the Nautilus from the examination of 

 several specimens preserved in spirit ; and, further, I can assure 

 M. Favre that the edge of the mantle in these animals is 

 iquite free from the edge of the shell, and that the chambers 

 of these shells are formed in the same way as the septa in 

 other shells — as, for example, the septa across the vertex of 

 BuUnius decurtatus and other decurtated shells. 



I am willing to allow that there are things to be explained 

 in regard to the formation of the septa and the siphons and 



