NAUTILUS. 



A. Spiral, rounded, with contigaous whorls. 

 (Plate XII. fig. 2.) 



B. Spiral, rounded, with separated whorli. 

 (fig. 3.) 



C. Elongated and straightish. 



Shell univalve, convolute, smooth, many-chanm- 

 bered, the divisions perforated and connected by 

 a continued siphunculus or pipe, formed of a thin, 

 testaceous matter, and lined with a membrane of 

 the animal. The dissepiments are convex inwardly, 

 and the chambers become gradually larger from 

 the tip J in the last or external one of which the 

 animal is supposed to fix his habitation, keeping 

 up a communication with his interior apartments 

 by means of the hollow tube which passes through 

 them all. 



This and the preceding genus have many natu- 

 ral characteristics common to both ; but they are 



