MOLLUSCA. 



however, there may be detected a tendency to the 

 spiral form at the apex of the cone. The broad foot 

 adheres so firmly to the rocks on which they are 

 found, that it is very difficult to remove without 

 destroying them ; while in some localities they occur 

 in such numbers, that the rock seems almost covered 

 by them. 



Somewhat allied to these, in general form and ap- 

 pearance, is the genus 



Chiton,* the Coat of Mail, 



but which shews a remarkable exception to the 

 usual univalve construction of the shells in this 

 Class. Instead of a single shell, we find a shield 

 composed of several plates overlapping each other 

 down the back, and surrounded by the broad un- 

 covered edge of the tough and leathery mantle, in 

 which they are partly imbedded. Like the Limpets, 

 they attach themselves to rocks, often above low- 

 water mark, so that they are left dry at intervals, and 

 not unfrequently they are found adhering to the bot- 

 toms of ships, returned from tropical voyages. 



" When we consider," says Mr. Kirby, " that 

 these animals are not only often exposed to the 

 violent action of the waves, but also to the attack 

 of countless enemies, we see abundant reason for 

 the coat of mail, with which their Creator has cover- 

 ed them. Even the fleshy or cartilaginous margin 

 or zone, is defended sometimes by scales, spines, 



* Xiruv, chiton, a coat. 



