72 Lamarck's Genera of Shells. 



essentially so, actually belong to cephalopodous mollusca, and are 

 more or less internal. 



Lamarck divides the cephalopoda into three sections. 



First Section. Testaceous, polythalamous cephalopoda. Shell 

 multilocular, subinternal. 



Second Section. Testaceous, monothalamous cephalopoda. 

 Shell unilocular, wholly external. 



Third Section. Naked cephalopoda. No shell, either internal or 

 external. 



Section I. 

 Polythalamous Cephalopoda. 



Shell multilocular, wholly or partly enveloped, inserted in the 

 posterior part of the body of the animal, often adhering. 



It appears that the shell of the polythalamous cephalopoda con- 

 tains the posterior part of the body of the animal, or a portion of 

 that part, in its last chamber ; but the shell itself is incased in the 

 posterior extremity of the body, and either entirely or partially 

 covered by it. 



In the spirula, about a fourth part of the shell is visible, or 

 exterior to the body of the animal. In the nautilus, probably, two- 

 thirds of the shell are uncovered, the rest being enveloped by the 

 posterior part of the Cephalopoda. 



The nummulites, and the other minute multilocular shells are, 

 on the contrary, probably wholly enveloped and hidden by the 

 posterior part of the animal which produces them ; and, perhaps, 

 the same may be the case with the ammonites, although many of 

 those shells are of very large size. 



Some of the animals of this section appear to contain their shell 

 without adhering to it, whilst others adhere by means of a ten- 

 dinous filiform ligament lodged in a sheath, which traverses the 

 chambers of the shell, and which increases in length in proportion 

 as the animal displaces the enveloped portion of its body; for, as 

 the animal grows, the last chamber of the shell must become too 

 small for the part contained in it ; it, therefore, probably withdraws 

 that part to some distance from the last chamber, leaving a void 



