MOLLUSCA OR UNIVALVES. 263 



Of those left, Spirula, Nautilus, and Argo- 

 nauta are perhaps the most interesting to the 

 young conchologist, and will therefore be fully 

 described. The remainder are nearly all fossils. 

 Some of the generic names have been changed 

 by modern naturalists; but as there is much 

 uncertainty respecting them, they will only be 

 described in a cursory manner. 



Division I. — Polythalama.* 



126. Belemnites. — Shell straight, in the 

 shape of a long cone; external part a sheath 



having a conical cavity in the upper part, and 

 containing an inner cone, which is chambered 

 and perforated. — 2 species described. 



These shells are only found in a fossil state, 

 and are very common, having been met with 

 in all ages and countries, and having given rise 

 to much speculation as to their real character. It 



* Les Cephalopodes-polythalames, Lamarck. 



s 4 



