Chambered Shells : Epidermis of Shells. 215 



which from the resemblance of the horny ones to claws of animals, 

 was usually called by the Greeks "Ov^l, h\irrai, and by Pliny 

 Cavum. I have therefore for the present, at least, kept the name 

 of Draparuaud. 



^ 8. On the formation of chambered and decollated Shells. 



The chambered shells, as the Nautili, seem evidently to be 

 formed on the same principle as other shells ; and the chambers 

 are added after the formation of the shell, for the purpose of 

 holding the straight body of the animal ; the siphon being only a 

 shelly tube formed by an appendage at the end of the body. The 

 cause of the formation of the chambers appears to be, that as the 

 animal enlarges, the shell, which is inelastic, must consequently be 

 too small for it ; it therefore gradually lengthens and enlarges the 

 cone of the shell, and the body, which does not so much increase 

 in length as in diameter, becomes too short for the cavity of the 

 shell, and would consequently leave a space behind it, which is not 

 consistent with the usual economy of the MoUusca. This space is 

 therefore obliterated by a secretion from the base of the mantle. 

 But as filling up the whole with shelly matter would occasion the 

 animal to drag after it much useless weight, and thus prevent 

 its floating, it is therefore closed by a shelly diaphragm, leaving 

 the space behind filled with air, whicii from its lightness assists 

 the economy of these curious animals, by enabling them to float 

 with greater ease. A similar peculiarity takes place in several of 

 the Turritella;, and other shells where the spiral cone is very 

 long and slowly enlarges ; while in many shells the part beyond 

 the diaphragm falls off, and they by that means become truncated 

 or decollated, as in the Helix decollata, H. purpurea, the Melaniaj, 

 Cerithia, &c. &c. 



§ 9. On the " Epidermis'' of Shells. 



The external surface of shells is generally covered with a kind 

 of web, that has been usually, from a fancied analogy to the cuticle 

 of the body, or the epidermis of plants, been called Epidermis. 

 Lamarck observing the error of the name, which carried with it 



