u 



NAUTILUS. 



A. Spiral, rounded, with contiguous whorls. 



(Plate XII. Fig. 2.) 



B. Spiral, rounded, with separated whorls. 



(Fig. 3.) 



C. Elono-ated and straig-htish. 



Shell univalve, convolute, smooth, many-cham- 

 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 ; 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 

 distinctly separated by the circumstance of one 

 being: concamerate. the other not so. The inhabit- 



