SHELLS ALLIED TO NAUTILUS. 273 



It is a curious fact, that contrivances, similar to those 

 which existed in some of the most early forms of Ammonite, 

 should have been again adopted in some of the most recent 

 species of fossil Nautili, in order to afford similar compensa- 

 tion for weakness that would otherwise have been produced 

 by aberrations from the normal structure of the genus Nau- 

 tilus. All this seems inexpUcable on any theory which 

 would exclude the interference of controlling Intelligence^ 



SECTION YI. 



CHAMBERED SHELLS ALLIED TO NAUTILUS AND AMMONITE. 



We have reason to infer, from the fact of the recent N. 

 Pompilius being an external shell, that all fossil shells of the 

 great and ancient family of Nautili, and of the still more 

 numerous family of Ammonites, were also external shells, 

 enclosing in their outer chamber the body of a Cephalopod- 

 We farther learn, from Peron's discovery of the shell of a 

 Spirula partially enclosed within the body of a Sepia,* (see 

 PI. 44, Fig. 1, 2,) that many of those genera of fossil cham- 

 bered shells, which, like the Spirula, do not terminate exter- 

 nally in a wide chamber, were probably internal, or partially 

 enclosed shells, serving the office of a float, constructed on 

 the same principles as the float of the Spirula. In the class 

 of fossil shells thus illustrated by the discovery of the animal 

 enclosing the Spirula, we may include the following extinct 

 families, occurring in various positions from the earliest 

 Transition strata to the most recent Secondary formations:. 



ia which the fringed edge is partially introduced, on the descending oi 

 inward portions only, of the lobated edge of tlie transverse plates. 



* The uncertainty which has arisen respecting the animal which con- 

 structs the Spirula, from the circumstance of the specimen discovered by 



