MOLLUSCA OR UNIVALVES. 267 



in great numbers, floating upon the surface 

 of the waves, with the head put out, and the 

 tentacula resting upon the water, the shell 

 at the same time being undermost; they do 

 not, in a general way, remain long sailing in 

 this manner, and have the power of easily re- 

 turning to their situation at the bottom of the 

 sea, by merely drawing in their tentacula and 

 upsetting the shell, which immediately sinks 

 with them. It was only in 1829 that this 

 animal was known with any certainty, one 

 having been caught alive, by Mr. G. Bennett, 

 near the New Hebrides Islands ; the animal, 

 after drawings were made of it, was preserved 

 in spirits, and is now in the museum of the 

 College of Surgeons. 



The ancients fabricated drinking-cups of the 

 shell of the Nautilus pompilius ; the other spe- 

 cies, the Umbilicatus, is very rare, and is distin- 

 guished by a large umbilicus on each side : the 

 former is often found in a fossil state. 



131. Ammonites.* — Shell discoid, spiral; 



* This genus has been divided into three ; Ammonites, 

 Goniatites, and Clymenia. 



