6 CATECHISM OF 



that when turned on its back, can recover its feet ; 

 the substance called tortoise shell is made from the 

 shell of this species. 



CHAPTER XVII. 



The same Subject continued. 



Q. How are Testaceous Fish divided ? 



A. Into 1. Univalves; 2. Bivalves; 3. Multi- 

 valves. 



Q. How many species of Univalves are there ? 



A. There are nineteen ; the names of which we 

 will give for the sake of reference to larger works ; 

 Argonauta, Nautilus, Conus, Cypraea, Bulla, Vo- 

 luta, Buccinum, Strombus, Murex, Trochus, 

 Turbo, Helix, Nerita, Haliotis Patella, Dentalium, 

 Serpula, Teredo, Sabella. A few observations on 

 the Nautilus and Murex, are all that our limits 

 will allow. 



Q. What is worthy of observation in the Nau- 

 tilus ? 



A. There are two species of Nautilus, the thick 

 and the thin shelled ; the former is, and the latter 

 is not fastened to its shell, but can leave it and re- 

 turn to it at pleasure ; the thin shelled species is 

 sometimes seen floating on the surface of the ocean 

 and expanding two arms as sails, while it uses two 

 others as oars ; when alarmed, it instantly sinks 

 to the bottom; the inside of the shell of the other 



